UC-NRLF 


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WIRELESS   TELEGRAPHY 

AND 

WIRELESS  TELEPHONY 


WIEELESS  TELEGEAPHY 

AND 

WIEELESS  TELEPHONY 

AN   ELEMENTARY  TREATISE 


BY 


A.  E.  KENNELLY,  A.M.,  Sc.D. 

Professor  of  Electrical    Engineering  In 
Harvard   University 


WITH  EIGHTY-FOUR  ILLUSTRATIONS 


NEW    YORK 
MOFFAT,  YARD  AND  COMPANY 


COPYRIGHT,  1906,  1909,  BY 
MOFFAT,  YARD  &  COMPANY 

Published,  November,  tgob 


Reprinted  with  additions,  February,  1909 
Reprinted,  May,  1909,  February,  1910 


PREFACE  TO  SECOND  EDITION. 

Since  the  first  edition  of  this  book  went  to 
press,  much  development  has  taken  place  in 
wireless  telegraphy.  The  range  distance  to 
which  messages  have  been  carried  has  in- 
creased. The  number  of  ship  and  shore  sta- 
tions has  approximately  doubled.  The  num- 
ber of  wireless  messages  transmitted  has 
greatly  increased.  A  very  notable  service  re- 
cently rendered  by  wireless  telegraphy  has 
drawn  public  attention  forcibly  to  its  value  as 
a  means  of  protecting  lives  at  sea.  Early  on 
the  morning  of  January  23rd,  1909,  the  east 
bound  White  Star  Liner  "  Republic"  was  in- 
jured by  collision  with  the  west  bound  Italian 
Liner  "  Florida,"  about  fifteen  miles  south  of 
Nantucket  light  ship,  in  a  dense  fog.  A  hur- 
ried wireless  general  call  for  assistance  brought 
several  vessels  to  the  rescue  and,  in  particular, 
the  White  Star  Liner  "  Baltic,"  that  happened 
to  be  sixty-four  nautical  miles  west  of  the  scene 
of  accident.  So  dense  was  the  fog  that  the 


271151 


PREFACE 

Baltic  steamed  for  twelve  hours  over  a  zigzag 
course  of  some  two  hundred  nautical  miles  be- 
fore reaching  the  helplessly  injured  and  drift- 
ing vessel.  Even  then  the  search  would  prob- 
ably have  been  futile,  if  wireless  messages  be- 
tween the  ships  and  the  shore  station  at  Sia- 
conset  had  not  assisted  the  meeting.  About 
sixteen  hundred  persons  were  ultimately  trans- 
ferred to  the  Baltic  from  the  Republic,  which 
shortly  afterwards  foundered  in  deep  water. 
No  loss  of  life  occurred  except  in  the  actual 
collision. 

In  spite,  however,  of  the  above  achievements 
of  wireless  telegraphy  during  the  past  two 
years,  the  development  of  wireless  telephony 
has  been  still  more  pronounced.  During  that 
time,  the  range  distance  of  wireless  telephony 
has  remarkably  increased;  so  that,  although 
far  below  that  of  wireless  telegraphy  of  the 
present  time,  it  is  comparable  to  that  of  wire- 
less telegraphy  ten  years  ago.  It  has,  there- 
fore, seemed  desirable  to  add  to  this  book  sev- 
eral chapters  on  wireless  telephony,  while 
bringing  the  subject  of  wireless  telegraphy  up 
to  date. 

Wireless  telephony  differs  from  wireless 
telegraphy  in  details  rather  than  in  fundamen- 


PREFACE 

tals ;  but  as  an  achievement  of  the  human  race, 
the  transmission  of  the  voice  to  great  distances 
on  the  ripples  of  electromagnetic  waves  is,  in 
one  sense,  a  far  greater  extension  into  free 
space  of  the  range  of  individual  personality 
than  any  form  of  wireless  telegraphy  thus  far 
attempted. 

Although  it  has  been  the  endeavor  to  pre- 
sent to  the  reader  the  fundamental  and  es- 
sential principles  both  of  wireless  telegraphy 
and  wireless  telephony,  rather  than  any  de- 
scription of  particular  systems  or  inventions; 
yet  in  treating  of  wireless  telephony,  much  has 
been  taken  directly  from  the  publications  of 
Prof.  R.  A.  Fessenden,  who  has  done  so  much 
to  extend  the  knowledge  and  practice  of  wire- 
less telephony  in  America. 

The  author  also  desires  to  express  his  in- 
debtedness to  the  writings  of  Messrs.  B.  S. 
Cohen,  G.  M.  Shepherd,  and  Ernst  Ruhmer. 

Cambridge,  Mass.,  February,  1909. 


CONTENTS 

CHAPTER  PAGE 

I  INTRODUCTORY    i 

II  WAVES  AND  WAVE-MOTION 3 

III  MAGNETISM  AND  ELECTRICITY  ....  14 

IV  ELECTROMAGNETIC  WAVES  GUIDED  OVER 

CONDUCTORS 29 

V    ELECTROMAGNETIC    WAVES    GUIDED   BY 

SINGLE  WIRES 36 

VI    RADIATED     ELECTROMAGNETIC      WAVES 

GUIDED  BY  THE  GROUND 40 

VII    UNGUIDED    OR   SPHERICALLY    RADIATED 

ELECTROMAGNETIC  WAVES    ...      .      64 
VIII    PLANE  ELECTROMAGNETIC  WAVES       .     .      75 
IX    THE  SIMPLE  ANTENNA  OR  VERTICAL  ROD 

OSCILLATOR 97 

X    ELECTROMAGNETIC  WAVE-DETECTORS  OR 

WIRELESS  TELEGRAPH  RECEIVERS  .     .     124 

XI    WIRELESS  TELEGRAPH  WORKING  ...     149 

XII    TUNED  OR  SELECTIVE  SIGNALING    ...     173 

XIII  MEASUREMENTS     OF    ELECTROMAGNETIC 

WAVES 191 

XIV  INDUSTRIAL  WIRELESS  TELEGRAPHY  .     .  198 
XV    CONSIDERATIONS  PRELIMINARY  TO  WIRE- 
LESS TELEPHONY 209 

XVI    THE  PRINCIPLES  OF  WIRE  TELEPHONY  .    225 

XVII    PRINCIPLES  OF  WIRELESS  TELEPHONY     .    239 

vii 


CHAPTER  I 

INTRODUCTORY 

ELECTRICITY  is  not  in  its  infancy,  populai 
impression  notwithstanding.  Electric  applica- 
tions, such  as  the  telephone,  the  wire  telegraph, 
the  electric  lamp  and  the  electric  motor,  are  very 
familiar  in  modern  life  and  have  been  for  a  num- 
ber of  years.  Electricity  has  reached  adoles- 
cence in  these  directions.  But  wireless  teleg- 
raphy, the  most  recent  of  electric  applications, 
is,  perhaps,  in  its  infancy.  It  is  only  about  ten 
years  old. 

There  is  something  fascinating  about  the  way 
in  which  electricity  works.  It  is  as  swift  as  it  is 
stealthy.  Electric  impulses  move  over  wires  at 
enormous  speeds  and  yet  the  action  is  invisible 
and  inaudible,  appealing  to  no  sense  directly. 
A  telegraph  wire  runs  overhead  say  from  New 
York  to  Buffalo  and  the  New  York  sending 
operator  closes  the  circuit  at  his  key.  Almost 
instantly,  and  certainly  within  about  one-tenth 
of  a  second,  the  little  lever  of  the  receiving  in- 
strument at  Buffalo  responds.  The  electric  im- 


2  WIRELESS  TELEGRAPHY 

pulse  reaches  its  destination  perhaps  as  rapidly 
as  we  can  turn  our  thoughts  from  one  end  of  the 
wire  to  the  other. 

We  cannot  see  the  electric  impulse  rush  along 
the  wire,  we  cannot  hear  it  travel.  We  can  only 
picture  the  transfer  in  our  imagination.  We  can 
see  the  wire  and  we  know  it  travels  along  that. 
If  we  cut  the  wire  anywhere,  the  electric  current 
is  stopped. 

When  we  turn  to  wireless  telegraphy,  how- 
ever, we  are  deprived  even  of  the  consolation  of 
a  guiding  wire  to  aid  the  imagination.  The 
phenomenon  of  the  wire  telegraph  is  a  mystery, 
but  a  familiar  one  to  which  the  wire  is  a  clue. 
The  new  phenomenon  of  the  wireless  telegraph 
is  a  yet  more  elusive  mystery  with  no  clue,  at 
first  sight.  Nevertheless,  we  shall  see  in  the 
sequel  that  there  is  not  so  much  difference 
between  the  two  cases  as  might  at  first  be  sup- 
posed. The  relations  between  wireless  teleg- 
raphy and  wire  telegraphy  resemble  the  rela- 
tions between  sound  distributed  in  the  open  air, 
and  sound  channeled,  or  confined,  within  a  speak- 
ing tube. 


CHAPTER  II 

WAVES   AND   WAVE-MOTION 

SINCE  wireless  telegraphy  is  carried  on  by  means 
of  electro-magnetic  waves,  it  is  desirable  to  ex- 
amine into  the  nature  of  those  types  of  waves 
with  which  we  are  more  familiar,  before  taking 
up  the  consideration  of  the  less  familiar  electro- 
magnetic waves. 

Free  Ocean  Waves 

A  wave  is  a  progressive  disturbance,  or  a  dis- 
turbance which  moves  along  through  some  kind 
of  medium.  The  most  familar  type  of  wave  is 
the  disturbance  in  the  level  of  water  by  some 
displacing  force,  such  as  wind,  or  the  splash 
of  a  falling  body.  Even  in  calm  weather,  we 
usually  find  a  wave-motion,  commonly  called 
swell,  on  the  surface  of  the  ocean.  We  then  find 
displacements  of  the  ocean  level,  alternately  up 
and  down,  or  high  and  low,  moving  along  the 
surface,  These  waves  have  a  certain  direction, 
say  from  west  to  east,  in  the  horizontal  plane 
or  plane  of  ocean  level.  They  also  have  a  cer- 
3 


4  WIRELESS  TELEGRAPHY 

tain  speed  in  this  direction.  On  the  deep  ocean 
this  speed  may  be,  say,  12  meters  per  second 
(39.4  feet  per  second,  or  26.8  miles  per  hour). 
Of  course  this  does  not  mean  that  in  calm  weather 
a  cork,  or  lifeboat,  floating  on  the  sea,  is  moved 
along  by  the  swell  at  this  high  speed.  The  cork, 
or  the  boat,  bobs  up  and  down  on  the  swell,  with 
only  a  very  small  movement  in  the  swell's  direc- 
tion. But  it  means  that  a  torpedo  boat  would 
have  to  steam  in  this  case  at  a  speed  of  26.8  miles 
per  hour  from  west  to  east  in  order  to  keep 
abreast  of  any  one  particular  roller  in  the  swell. 
It  is  to  be  observed  that  the  waves  advance 
through  and  over  the  water,  without  carrying  the 
water  bodily  along  to  any  marked  extent. 

Another  noteworthy  point  in  the  waves  of 
ocean  swell  is  that  there  is  commonly  a  fairly 
definite  and  uniform  wave-length.  This  wave- 
length is  measured  in  the  direction  of  wave- 
motion,  and  may  be  taken  as  the  distance  from 
crest  to  crest  of  successive  waves.  This  wave- 
length in  open  ocean  swell  is  often  about  100 
meters  (328  feet  or  109.3  yards).  A  tightly 
stretched  string  328  feet  long,  held  east  and 
west,  would  in  such  a  case  just  span  the  de- 
pression between  successive  roller  crests. 

The  waves  of  the  ocean  manifestly  contain 
energy.  That  is  to  say,  work  had  to  be  done 


WAVES  AND  WAVE-MOTION  5 

originally  by  the  winds  to  produce  them,  and 
the  waves  are  capable  of  doing  a  good  deal  of 
work  before  being  brought  to  rest.  Attempts 
have  been  made,  in  fact,  to  obtain  work  from 
waves  at  suitable  points  on  the  sea-coast  by 
means  of  engines  operated  by  the  rise  and  fall 
of  the  waves. 

Sound- Waves  in  Air 

A  type  of  wave  of  which  we  are  constantly 
receiving  impressions  through  our  ears,  but 
which  is  more  difficult  to  analyze  than  the  ocean 
wave,  is  the  sound-wave  in  the  atmosphere. 
Waves  of  sound  are  invisible  and  hence  the 
difficulty  we  experience  in  becoming  familiar 
with  their  forms,  speed  and  other  properties. 
We  learn  that  sound  in  air  is  a  disturbance  in 
its  density  and  pressure  which  moves  through 
the  air  at  a  definite  speed.  If  we  fire  a  pistol  in 
the  air,  the  explosion  in  the  barrel  displaces  the 
air,  or  compresses  it,  in  the  immediate  neigh- 
borhood of  the  discharge.  The  zone  of  com- 
pression moves  off  into  surrounding  air  with 
substantially  the  same  speed  in  all  directions,  if 
the  air  is  calm,  and  is  followed  immediately  by 
a  zone  of  expanded  air;  just  as  a  hollow  or  de- 
pression follows  a  hump  or  elevation  in  an  ocean 
wave. 


6  WIRELESS  TELEGRAPHY 

Fig.  i  gives  a  diagrammatic  view  of  a  single 
sound-wave  of  compression  and  dilation,  shortly 
after  the  wave  has  moved  off  from  the  explosion 


FIG.  i.  —  Diagram  Indicating  a  Single  Sound  Wave 
Shortly  after  Leaving  Its  Source. 

or  origin  of  the  disturbance,  o.  The  wave  is 
hemispherical  in  form,  and  the  diameter  of  the 
hemisphere  is  a  a,  at  the  instant  considered. 
The  external  portion  of  the  hemispherical  wave 
shell  contains  compressed  air,  indicated  by  the 
concentric  semi-circles.  The  internal  dotted 
portion,  immediately  following,  contains  ex- 
panded air.  After  a  brief  interval  of  time  the 
wave  will  have  expanded  to  the  contour  indi- 
cated in  Fig.  2,  where  the  diameter  of  the  hemis- 


FIG.  2. — Diagram  Indicating  a  Single  Sound  Wave 
Which  Has  Separated  Itself  from  Its  Origin  by 
Five  Wave-Lengths. 

pherical  shell  is  b  o  b.  The  length  of  the  wave 
is  the  distance  b  c,  measured  radially  from  the 
front  to  the  back  of  the  wave,  or  from  the  out- 


WAVES  AND  WAVE-MOTION  7 

side  to  the  inside  of  the  hemispherical  shell.  If 
this  wave-length  be,  say  100  meters  (328  feet  or 
109.3  yards),  then  the  distance  o  c,  by  which 
the  wave  has  already  removed  itself  from  the 
origin,  is  500  meters  (1640  feet  or  546.7  yards). 
Another  brief  interval  of  time  would  bring  about 
the  condition  indicated  in  Fig.  3,  where  the  wave 


cd 


FIG.  3. — Diagram  Indicating  a  Single  Sound- Wave 
Which  Has  Separated  Itself  from  Its  Origin  by 
Eleven  Wave-Lengths. 

has  expanded  hemispherically  to  the  diameter 
c  o  c.  The  length  of  the  wave,  c  d  remains  as  it 
was  in  the  earlier  stages,  assumed  as  100  meters 
(328  feet  or  109.3  yards);  but  the  distance  o  d 
which  the  wave  has  now  placed  between  itself 
and  the  origin  is  uoo  meters  (3609  feet  or  1203 
yards).  Observers  in  balloons  floating  in  the 
air  at  points  such  as  e  or  /  on  the  wave  front, 
would  hear  the  sound  of  the  explosion  simul- 
taneously with  observers  on  the  ground  at 
points  c  c. 


8  WIRELESS  TELEGRAPHY 

Speed  oj  Sound-Waves 

The  speed  with  which  the  sound-waves  moves 
radially  outward  in  all  directions  is  approxi- 
mately 333  meters  per  second  (1090  feet  per 
second,  or  746  miles  per  hour),  depending 
slightly  upon  the  temperature  and  humidity  of 
the  air.  We  Cannot  see  the  hemispherical  shell 
of  disturbed  air  expanding;  but  we  can  picture 
the  process  to  the  mind's  eye.  In  an  hour,  the 
expansion  would  carry  the  radius  of  the  hemis- 
phere to  a  distance  of  1200  kilometers  (746 
miles).  But  the  density  of  the  atmosphere 
would  be  infinitesimally  small  at  an  elevation 
amounting  to  this  distance,  and  sound  being 
a  disturbance  of  air  cannot  travel  where  the  air 
ceases  to  exist.  Consequently,  the  hemispheri- 
cal form  of  the  wave  must  disappear  at  great  dis- 
tances, for  lack  of  air  above  the  origin 

Dilution  of  Intensity  in  Sound- Waves 

The  energy  residing  in  the  sound-wave  would 
be  the  same  in  the  successive  states  of  Figs,  i,  2, 
and  3,  if  there  were  no  expenditure  of  energy  in 
friction  during  the  motion.  That  is  to  say,  we 
may  suppose  that  a  certain  part  of  the  energy  in 
the  explosion  at  o  was  stowed  away  in  the  wave 
of  compressed  and  dilated  air.  But  the  space 


WAVES  AND  WAVE-MOTION  9 

occupied  by  the  wave  in  the  stage  of  Fig.  2  with 
radius  o  a,  150  meters  (164  yards),  would  be  13.6 
millions  of  cubic  meters;  in  the  stage  of  Fig.  2, 
382  millions  of  cubic  meters,  and  in  the  stage  of 
Fig.  3,  1660  millions  of  cubic  meters  (corres- 
ponding to  17.8,  500,  and  2160  millions  of  cubic 
yards  respectively).  It  is  evident,  therefore, 
that  the  energy  in  the  wave  is  constantly  spread 
out  into  more  space,  or  diluted,  as  the  wave 
expands;  so  that  the  energy  in  a  given  volume, 
such  as  i  cubic  meter,  is  constantly  diminishing. 
This  is  another  way  of  saying  that  the  intensity 
of  the  wave  diminishes  as  time  goes  on,  and  the 
radius  of  the  wave  increases.  The  loudness  of 
the  explosion  as  noted  by  an  observer  at  a  in 
Fig.  i  would  be  considerably  greater  than  that 
noted  by  an  observer  at  b  in  Fig.  2;  or  again, 
than  that  noted  by  an  observer  at  c  in  Fig.  3. 

If  we  were  to  place  a  sufficiently  sensitive  re- 
cording barometer  anywhere  in  the  neighbor- 
hood of  the  explosion,  and  carefully  observe  the 
barometer  record  as  the  noise  of  the  explosion 
occurred,  we  should  expect  to  find  that  the  barom- 
eter would  record,  just  before  the  explosion,  a 
horizontal  straight  line  a  b,  Fig.  4,  corresponding 
to  the  reading  of  the  barometer  at  that  time,  say 
760  millimeters  (29.92  inches)  of  mercury. 
When  the  sound  of  the  explosion  arrived,  the 


io  WIRELESS  TELEGRAPHY 

barometer  would  rise  very  slightly  to  b  c,  then 
fall  to  e,  and  then  return  to  the  normal  straight 
line  /  g.  The  elevation  from  b  to  c  would  mark 
the  degree  of  compression  in  the  first  half  of  the 
wave,  and  the  depression  from  d  to  e  would 
similarly  mark  the  following  dilatation.  The 
crest  height  k  c,  or  kt  e,  would  measure  the 
amplitude  of  the  disturbance  or  amplitude  of  the 
wave.  If  the  barometer  were  located  close  to 


FIG.  4. — Ideal  Barometric  Record  at  the  Time  of  Pas- 
sage of  the  Noise  of  an  Explosion. 

the  origin  of  the  explosion,  as  at  a  in  Fig.  i,  the 
amplitude  of  the  pressure  disturbance  record, 
and  the  amplitude  of  the  recorded  sound,  would 
be  relatively  large.  If,  on  the  other  hand,  the 
barometer  were  placed  further  from  the  origin, 
as  at  c  in  Fig.  3,  the  amplitude  both  of  the 
recorded  disturbance  and  of  the  sound-wave  at 
the  barometer  would  be  smaller. 

Ordinary  sound-waves  possess  so  little  energy, 
or  have  so  small  an  amplitude,  that  recording 
barometers  show  no  trace  of  them.  Expressing 
the  same  thing  in  another  way,  the  impression- 


WAVES  AND  WAVE-MOTION  n 

producing  mechanism  of  the  ear  is  far  more 
sensitive  to  the  disturbances  of  pressure  in  sound- 
waves than  the  ordinary  barometer. 

Very  powerful  explosions  are  capable  of  pro- 
ducing sound-waves  of  sufficient  intensity  to  be 
observed  at  great  distances.  The  great  ex- 
plosion of  the  volcano  Krakatoa,  near  the  Sunda 
Straits,  in  the  year  1883,  is  stated  to  have  been 
heard  at  distances  greater  than  3200  kilometers 
(2000  miles).  The  outgoing  wave  affected  baro- 
meters all  over  the  world,  and  left  traces  on 
recording  barometers.  This  wave  is  stated  to 
have  traveled  at  a  speed  of  1130  kilometers  per 
hour  (700  miles  per  hour),  to  have  swelled  at  the 
antipodes  to  Krakatoa,  in  18  hours,  and  to  have 
spread  out  again  over  the  globe.  It  was  not 
finally  lost  sight  of  until  it  had  passed  around 
the  globe  several  times.  When  the  first  out- 
going wave  passed  over  Singapore,  a  port  dis- 
tant about  830  kilometers  (516  miles)  from 
the  origin  of  the  disturbance,  the  gas-holder 
of  the  town  is  stated  to  have  leaped  into  the 
air  several  feet  up  and  down. 

Sound- Wave  Trains 

If  instead  of  producing  a  solitary  explosion  at 
the  origin,  and  a  corresponding  solitary  wave 
moving  off  radially  therefrom,  we  produce  a 


12 


WIRELESS  TELEGRAPHY 


rapidly  and  rhythmically  repeated  disturbance; 
as  in  blowing  air  through  an  organ-pipe,  or 
forcing  air  through  a  syren-wheel,  a  succession 
of  waves  is  produced,  and  the  sensory  effect 
produced  on  the  ear  is  that  of  a  tone  or  musi- 


FIG.  5. — Diagram  Representing  the  Succession  of  Sound- 
Waves  Emitted  from  the  Origin  O  When  a  Simple 
Musical  Note  Is  There  Produced. 

cal  note.  A  succession  of  outgoing  waves  is 
diagrammatically  represented  in  Fig.  5.  The 
shaded  areas  there  correspond  to  zones  of  com- 
pressed air,  and  the  intermediate  unshaded 
areas  to  zones  of  dilated  air.  Eight  complete 
waves  are  indicated.  If  the  note  sounded  be 
the  deepest  E  of  the  double-bass  viol,  making 
40  complete  vibrations  a  second,  then  the  length 


WAVES  AND  WAVE-MOTION  13 

of  each  wave  will  be  the  fortieth  part  of  333 
meters,  the  distance  which  sound  travels  in  a 
second,  because  40  complete  waves  will  occupy 
the  space  covered  by  advancing  sound  in  one 
second.  Each  wave  will,  therefore,  be  8.33 
meters  (27.34  feet  or  9.11  yards)  in  length,  and 
the  length  o  d  of  eight  wave-lengths  will  have 
been  covered  in  one-fifth  of  a  second  from  the 
commencement  of  the  sound. 

The  curve  at  D  o  or  o  D'  gives  the  trace-record 
that  we  should  expect  a  recording  barometer 
would  give  at  d  after  all  of  the  eight  waves 
passed  by,  on  the  supposition  that  this  pure, 
musical  note  was  sustained  uniformly  for  eight 
complete  cycles  of  the  disturbance.  If,  however, 
the  string  of  the  double-bass,  instead  of  being 
excited  by  the  bow,  were  plucked  by  the  finger 
in  such  a  manner  that  the  vibrations  died  away, 
then  the  record  of  the  supposed  sensitive  barom- 
eter at  d  might  indicate  the  curve  d,  o  of  de- 
caying amplitudes. 


CHAPTER  III 

MAGNETISM    AND    ELECTRICITY 

HAVING  paved  the  way  for  a  consideration  of 
electromagnetic  waves  by  a  few  outlines  of  sound 
waves  in  air,  we  may  now  fitly  turn  attention  to 
magnetism  and  electricity. 

Wind  and  Its  Energy 

Everyone  is  familiar  with  the  fact  that  wind  is 
an  active  or  disturbed  state  of  the  atmosphere,  a 
movement  of  the  air.  We  ordinarily  understand 
wind  to  be  a  uniform  movement  of  the  air  in  any 
one  given  direction,  and  we  ordinarily  under- 
stand by  eddies  or  gusts,  twisting  or  vortical 
movements  of  the  air,  but,  in  general,  wind  may 
include  both  linear  movement  and  vortical  move- 
ment, since  one  cannot  occur  in  the  atmosphere 
without  involving  the  other.  The  material  for 
the  creation  of  a  wind  is  always  present,  for  this 
material  is  the  air  itself.  We  only  need  to  ener- 
gise the  air  in  a  particular  way,  to  make  it  move 
forward.  Energy  must  be  expended  in  pro- 
ducing a  wind?  and  energy  resides  in  the  wind. 

14 


MAGNETISM  AND  ELECTRICITY  15 

If  we  employ  a  hand-fan  to  produce  a  local 
breeze,  we  must  expend  muscular  energy,  or  do 
work  on  the  fan,  to  force  the  air  into  motion,  and 
the  air  once  set  in  motion  contains  energy  or  can 
do  work  by  moving,  for  example,  light  obstacles 
in  its  path.  Consequently,  we  may  say  that 
wind  is  air,  plus  energy  given  to  it  in  a  particular 
way. 

Air  is  a  material  fluid.  It  forms  an  ocean  on 
the  surface  of  this  earth,  and  we  live  at  or  near 
the  bottom  of  this  air-ocean.  Air  gravitates,  or 
pulls  upon  the  mass  of  the  earth.  Each  indi- 
vidual atom  of  air  gravitates,  and  the  sum  total 
of  all  the  individual  pulls  exerted  on  the  earth 
amounts  to  a  pressure  of  about  i  kilogramme 
per  square  centimeter  of  surface  (14.25  Ibs.  per 
sq.  inch). 

The  Invisible  Ether 

It  is  generally  believed  that  all  space,  includ- 
ing the  interior  of  solid  bodies,  is  permeated  by 
an  immaterial  fluid  called  the  universal  ether. 
The  ether  is  just  as  invisible  as  air.  Whether  it 
consist  of  matter  or  not,  it  is  immaterial  in  the 
sense  that  it  apparently  does  not  gravitate.  It 
does  not  directly  appeal  to  any  sense,  but  it  is 
much  easier  to  assume  its  presence  everywhere 
than  to  deny  its  existence.  If  we  take  a  vacuum- 


1 6  WIRELESS  TELEGRAPHY 

tube,  i.e.,  a  sealed  glass  tube  from  the  interior 
of  which  the  air  has  been  almost  entirely  re- 
moved, it  can  be  shown  experimentally  that 
sound  cannot  move  across  the  interior  of  the 
tube,  but  light  passes  across  it,  and  so  do 
radiant  heat  and  gravitational  force.  We  can- 
not believe  that  these  activities  are  transmitted 
through  absolutely  empty  space.  Something 
must  transmit  them,  for  they  are  transmitted 
at  definite  speeds.  This  something  is  named 
the  ether.  Beyond  its  powers  of  transmitting 
energy,  hardly  anything  is  yet  known  about  the 
ether.  Its  structure,  and  the  manner  in  which 
it  permeates  space,  are  still  unsolved  riddles. 

Nature  oj  Electricity  and  Magnetism 

As  soon  as  the  ether  is  postulated  to  be  a  uni- 
versal fluid  or  medium  in  which  all  matter  swims, 
so  to  speak,  many  things  may  be  accounted  for 
which  otherwise  we  could  not  even  attempt  to 
explain.  Electricity  and  magnetism,  for  ex- 
ample, may  be  accounted  for  in  a  general  way. 
Just  as  wind  is,  we  have  seen,  a  particular  ener- 
gized condition  of  the  circumambient  air,  so 
both  electricity  and  magnetism  are  particular 
energized  conditions  of  the  universal  ether,  which 
underlies  the  air  and  everything  else.  It  is  not 
BO  easy,  however,  to  define  the  nature  and  rela- 


MAGNETISM  AND  ELECTRICITY  17 

tions  of  these  particular  energized  conditions. 
We  cannot  at  present  say,  for  example,  that 
electricity  is  the  same  kind  of  motion  of  the 
ether  that  wind  is  of  the  air.  If  we  do  not  yet 
apprehend  the  nature  of  the  ether  itself,  how 
shall  the  task  be  undertaken  of  denning  its  ener- 
gized conditions?  The  energized  conditions 
might  be  statical,  and  involve  no  motion  of  ether, 
like  the  energy  of  a  stationary  coiled-up  spring; 
or  they  might  be  dynamical,  and  involve  modes 
of  motion  of  the  ether.  In  any  event,  it  seems 
clear  from  the  known  laws  of  electromagnetism 
that  there  is  a  definite  mutual  relation  between 
the  two  energized  conditions  of  ether,  electricity 
and  magnetism,  such  that  as  soon  as  either  is 
defined  the  other  also  is  immediately  determined. 
In  mathematical  language,  one  is  the  "curl"  of 
the  other.  If,  for  instance,  electricity  should  be 
a  definite  kind  of  tension  or  static  stress  long- 
wise, then  magnetism  would  be  a  definite  kind  of 
twist  or  crosswise  static  stress,  and  reciprocally. 
Or,  if  electricity  should  be  a  simple,  straight- 
forward motion,  or  streaming,  of  the  ether,  then 
magnetism  would  be  eddy  motion  or  vortical 
rotational  motion,  or  spin,  of  the  ether,  and  re- 
ciprocally. It  is  surprising  how  much  is  known 
concerning  the  laws  of  action  and  behavior  of 
electricity  and  magnetism,  considering  the  little 


1 8  WIRELESS  TELEGRAPHY 

that  is  known  of  their  absolute  fundamental 
nature.  We  can  control  electricity  and  magne- 
tism remarkably  well,  considering  that  we  do  so 
from  beyond  a  hitherto  impenetrable  veil  that 
does  not  admit  of  perceiving  the  things  directed. 
It  is  evident  that  whatever  may  be  the  precise 
nature  of  electricity  and  magnetism,  the  widely 
admitted  postulate  of  the  universal  ether  requires 
that  the  material  for  either  or  both  is  omnipresent. 
Just  as  the  material  for  wind  is  always  present 
in  the  circumambient  air,  and  all  we  need  is  the 
application  of  energy  to  the  air  in  a  particular 
way  in  order  to  produce  a  wind,  so  the  material 
for  electricity  or  magnetism  is  universally  present, 
and  all  we  need  is  the  application  of  energy  to 
the  ether  in  particular  ways.  Consequently, 
electricity  and  magnetism  may  be  regarded  as 
the  ether,  plus  particular  forms  of  energy. 

Magnetic  Flux  and  Its  Properties 

If  we  consider  an  ordinary,  permanent  horse- 
shoe magnet,  such  as  is  indicated  in  Fig.  6,  we 
find  that  all  around  it,  and  particularly  between 
its  poles  N  and  s,  there  is  a  certain  invisible  ac- 
tivity which  possesses  both  direction  and  inten- 
sity. In  the  illustration,  the  direction  is  roughly 
indicated  by  the  broken  lines  with  arrow-heads, 
and  the  intensity  by  the  relative  crowding  or  con- 


MAGNETISM  AND  ELECTRICITY 


densation  of  these  lines.  The  direction  of  this 
magnetic  activity  in  the  air  between  the  poles  is 
seen  to  be  from  the  north  pole  N  to  the  south 
pole  S.  This  is  strictly  speaking  a  pure  con- 
vention. It  might  have  been  originally  agreed 
to  draw  all  the  arrows 
in  the  opposite  direction. 
All  that  is  certain  is 
that  there  is  a  definite 
polarity  about  the  sys- 
tem, and  that  the  actions 
pertaining  to  the  north 
pole  are  distinctly  in- 
verse to  the  actions  per- 
taining to  the  south  pole, 
magneticians  all  agree-  \  ""-• — •'"  / 

ing  upon  the   direction  """*•. .*.••*'* 

shown.     The  north  pole 

.      -  ,        i  .  i      .r  ,,        FIG.  6. — Diagram  of  Mag- 

lS  the  pole  which,  if  the       netic  Flux  in  the  Space 

magnet  were  freely  sus-       Between  the  Poles  of  a 

•  Permanent  Magnet, 

pended,  would  seek  for, 

or  point  approximately  toward,  the  north  geo- 
graphic pole  of  the  earth,  or  the  earth's  magnetic 
pole  near  the  Greenland  end  of  the  earth's  axis. 
That  is,  the  N  end  is  the  north-seeking  end. 

As  roughly  indicated  in  Fig.  6,  the  magnetic 
activity,  or  magnetic  flux  as  it  is  called,  is  densest 
or  most  intense,  in  the  air  between  the  opposing 


20  WIRELESS  TELEGRAPHY 

pole-tips,  or  where  the  air-space  separating  the 
poles  is  shortest.  As  we  leave  this  region,  the 
magnetic  flux  becomes  thinner,  or  weaker.  A 
peculiarity  about  this  flux  is  that  it  always  re- 
turns back  upon  itself  in  closed  loops  or  chains. 
In  other  words,  magnetic  flux  is  always  con- 
tinuous and  re-entrant.  At  first  sight  it  appears 
to  be  discontinuous,  because  it  seems  to  com- 
mence at  one  pole  and  end  at  the  other.  But  it 
can  be  shown  experimentally  that  the  flux  con- 
tinues through  the  interior  substance  of  the  steel 
magnet,  and  each  loop,  such  as  N  A  S,  completes 
a  circuit  B  C  D  E  F  within  the  substance  of  the 
magnet. 

Provisional  Hypothesis  as  to  Nature  of  Magnetic 
Flux 

Although  the  real  nature  of  this  magnetic  flux 
is  not  yet  known,  yet  it  may  help  us  to  follow  the 
actions  of  electromagnetic  waves  later  on,  if  we 
assume,  for  the  purposes  of  description,  that 
magnetic  flux  is  a  streaming  motion  of  the  ether. 
On  this  assumption,  a  permanent  magnet  is  a 
force  pump  which  draws  the  ether  in  at  the 
south  pole,  through  the  substance  of  the  steel 
in  the  interior,  and  forces  it  out  at  the  north  pole. 
With  no  friction,  this  streaming  would  not  neces- 
sarily absorb  energy,  and  we  know  that  perma- 


MAGNETISM  AND  ELECTRICITY          21 

nent  magnets  may  be  designed  to  retain  their 
magnetism  without  sensible  diminution  for  an 
indefinitely  long  time. 

Energy  oj  Magnetic  Flux 

Although  magnetic  flux  does  not  need  energy 
to  be  expended  in  order  to  keep  it  going,  yet 
energy  has  to  be  expended  to  create  it.  That  is, 
magnetic  flux  contains  energy,  or  has  energy 
always  associated  with  it.  As  long  as  the  mag- 
neitc  flux  persists,  the  energy  resides  quiescent 
with  it.  When  the  flux  disappears,  its  energy 
disappears  also.  Consequently  work  must  be 
done  to  create  magnetic  flux,  and  magnetic  flux 
is  able  to  do  work  or  give  up  its  energy  when  it 
disappears. 

Between  the  opposed  pole-tips  N  S,  we  may 
consider,  on  the  above  hypothesis,  that  the 
stream  of  ether  is  densest,  and  receding  from  this 
region  the  stream  gets  weaker.  The  streaming 
is  steady  both  at  any  particular  point  for  all  con- 
sidered time,  and  for  all  points  at  any  one  time. 
The  magnet  ether-pump  is  steady.  The  pump- 
ing action  is  due  to  activities  in  the  molecules 
of  the  steel.  Each  molecule  of  iron  is  supposed 
to  be  a  little  individual  ether  pump,  by  virtue  of 
/hternal  activities  as  yet  only  dimly  guessed  at. 
When  the  horseshoe  is  magnetized,  all  of  the  mo- 


22  WIRELESS  TELEGRAPHY 

lecules  are  caused  to  align  themselves  in  parallel 
directions,  or  to  face  the  same  way,  whereby 
they  all  pump  the  ether  in  the  same  general  di- 
rection. Within  the  iron  molecules,  the  pump- 
ing activities  are  believed  to  be  electric;  but  into 
these  we  need  not  enter.  The  point  here  to  be 
observed  is  that  in  the  air-space  outside  of  the 
magnet,  the  steady  magnetic  flux  produces  no 
electric  action.  In  this  air-space  we  find  mag- 
netism but  not  electricity.  If,  however,  we  move 
the  magnet,  and  with  the  magnet  the  system  of 
magnetic  flux  pertaining  thereto,  there  will  be 
electric  action  produced  where  the  magnetic  flux 
lines  are  carried  through  space.  If,  for  example, 
the  magnet  be  lifted  bodily  toward  the  observer 
without  twisting,  feeble  electric  forces  will  be 
brought  into  play  in  directions  lying  within  the 
plane  of  the  horseshoe.  In  the  region  between 
the  poles  these  electric  forces  will  be  directed, 
during  the  motion,  in  the  direction  from  A  to  D. 
The  intensity  of  these  electric  forces  will  be  pro- 
portional to  the  speed  with  which  the  magnetic 
flux  moves  sidewise.  If  the  magnetic  flux 
moves  longwise,  or  parallel  to  itself,  there  is  no 
electric  action  set  up,  but  if  the  magnetic  flux 
moves  sidewise,  or  crabwise,  there  is  electric 
action  set  up.  It  is  on  this  action  that  all  dyna- 
mos depend;  namely,  upon  relative  sideways 


MAGNETISM  AND  ELECTRICITY  23 

motion  between  magnetic  flux  and  an  electric 
conductor  to  pick  up  and  utilize  the  induced 
electric  force.  Steady  and  stationary  magnetic 
flux  is  thus  unaccompanied  by  electric  action, 
but  unsteady,  varying,  or  sidewise-moving  mag- 
netic flux  sets  up  electric  action. 

Electric  Flux  and  Its  Properties 

Turning  now  to  electricity,  Fig.  7  represents 
a  vertical  metallic  rod,  and  terminal  balls,  sup- 


».  ••  -«-»•«.  Si  .->•*•»..  ^ 


* 
4 


FIG.  7. — Diagram  of  Electric  Flux  in  the  Space  Between 
an  Electrified  Rod  and  Disk. 

ported  by  an  insulating  holder  not  shown,  in  air 
above  the  center  O,  of  a  horizontal,  insulated 
metallic  disk  COD.  This  insulated  pair  of 
conductors  may  be  electrically  charged  either  by 
a  frictional  electric  machine,  an  influence  ma- 
chine, a  spark  coil,  or  a  voltaic  battery.  That 


24  WIRELESS  TELEGRAPHY 

is,  the  charge  may  be  communicated  from  any 
suitable  electric  source.  The  charge  will  be  re- 
tained, because  the  rod  is  insulated  from  the  disk, 
and  if  the  insulation  could  be  made  perfect,  the 
charge  would  be  retained  indefinitely.  Let  us 
suppose  that  the  rod  is  positively  charged  and 
the  disk  negatively. 

In  the  air-space  between  the  electrified  rod 
and  disk  there  is  an  invisible  influence  which 
possesses  both  direction  and  intensity.  It  is 
called  electric  flux.  This  electric  flux  is  dia- 
grammatically  represented  in  Fig.  7  by  the  little 
arrows  which  proceed,  by  convention,  from  the 
positive  charge  to  the  negative  charge.  The 
arrows  are  drawn  on  little  lines  of  points,  instead 
of  little  broken  lines  as  in  Fig.  6,  in  order  to  dis- 
tinguish them  from  lines  of  magnetic  flux.  Be- 
tween A  and  O,  where  the  air-space  is  shortest, 
the  electric  flux  is  most  densely  crowded,  or  its 
intensity  is  greatest.  As  the  separating  air- 
space increases,  the  flux  density  weakens. 

Energy  o]  Electric  Flux 

Energy  always  resides  in  the  electric  flux,  so 
that  each  and  every  part  of  the  region  permeated 
by  the  electric  flux  represented  in  the  illustration 
contains  energy.  The  energy  is  not  uniformly 
distributed.  It  is  greater  per  unit  volume  at  a 


MAGNETISM  AND  ELECTRICITY          25 

point  like  F  than  at  a  point  like  G.  It  is  stowed 
away  in  proportion  to  the  square  of  the  electric 
flux  density,  so  that  in  two  regions  one  of  which 
has  double  the  flux  density  of  the  other,  there 
will  be  four  times  more  energy  per  cubic  centi- 
meter, or  cubic  inch  of  space,  in  the  former  than 
in  the  latter.  As  long  at  the  electric  flux  per- 
sists, this  energy  resides  therein  or  accompanies 
it,  and  when  the  flux  disappears  the  energy  has 
disappeared.  This  energy  is  communicated  to 
the  ether  in  the  insulating  air  between  rod  and 
disk  at  the  time  of  their  charge. 

Provisional  Hypothesis  as  to  the  Nature  of 
Electric  Flux 

In  conformity  with  the  provisional  hypothesis 
already  adopted  for  magnetic  flux,  stationary 
electric  flux  may  be  assumed  to  be  an  elastic 
twist  or  stress  in  the  ether;  so  that  the  whole 
system  of  ether  tends  to  revolve  clockwise  about 
the  rod  A  B  as  axis,  when  looking  down  on  the 
disk  from  above.  The  screw  or  twist  will  have 
maximum  intensity  along  the  central  line  O  A, 
and  is  resisted  by  the  elastic  rigidity  of  the  ether. 
The  elastic  energy  of  the  twist  is  the  total  energy 
of  the  electric  flux  as  summed  up  throughout 
the  entire  electric  field,  or  permeated  insulating 


26  WIRELESS  TELEGRAPHY 

region.  The  amount  of  electric  energy  that  air 
can  be  made  to  hold  without  breaking  electrically, 
or  disrupting  into  a  spark  discharge,  depends 
upon  the  atmospheric  pressure  and  upon  the 
shape  of  the  opposed  electrified  surfaces.  At 
ordinary  atmospheric  pressures,  and  parallel 
opposed  surfaces,  the  most  favorable  form,  the 
energy  that  air  can  hold  is  limited  to  about  480 
ergs  per  c.  c.,  or  i  foot-pound  per  cubic  foot; 
i.e.,  the  work  done  by  lifting  one  pound  one  foot 
high,  to  the  cubic  foot  of  air  space  under  powerful 
electric  stress.  « 

Electric  flux  at  rest  differs  from  magnetic  flux 
at  rest  in  the  fact  that  the  former  is  discontinu- 
ous while  the  latter  is  continuous.  The  magnetic 
flux,  as  we  have  seen,  always  forms  closed  loops 
or  chains  in  space.  Steady  electric  flux,  on  the 
contrary,  always  starts  from  a  positive  charge 
and  ends  on  a  negative  charge.  In  the  case  of 
opposed  conductors,  the  charges  always  reside 
on  their  surfaces,  and  thus  the  electric  flux  al- 
ways starts  on  the  surface  of  the  positive  con- 
ductor and  ends  on  the  surface  of  the  negative 
conductor. 

Tensions  in  Electric  and  Magnetic  Fluxes 

The  electric  flux,  like  magnetic  flux,  always 
possesses  the  property  of  pulling  along  its  own 


MAGNETISM  AND  ELECTRICITY  27 

direction  at  the  same  time  that  it  pushes  side- 
ways. The  curved  arrow  lines  of  Fig.  7  merely 
indicate  the  direction  and  the  relative  crowding 
of  the  electric  flux  from  point  to  point  of  this 
particular  electrified  system;  but  if  we  suppose 
that  each  of  these  lines  is  a  little  elastic  thread, 
exerting  a  certain  mechanical  tension,  and  if  we 
also  suppose  that  each  such  elastic  thread  exerts 
a  repulsion  sideways  against  its  neighbors,  or 
tries  to  secure  all  the  elbow-room  it  can,  we  get 
an  idea  of  the  static  forces  which  reside  in  such 
a  stationary  electric  flux.  Thus,  the  line  H  G  K, 
in  addition  to  its  own  tension,  pushes  sideways 
against  the  adjacent  lines  h  g  k  and  L  M  D. 
The  resultant  effect  is  a  tension,  or  attractive 
pull,  between  the  rod  and  the  disk,  or  the  familiar 
attractive  force  between  oppositely  electrified 
bodies. 

As  long  as  the  electric  flux  remains  steady  and 
stationary,  no  magnetism,  or  magnetic  force  is 
produced.  There  will  be  a  tendency  to  move 
any  electrified  object,  such  as  a  pith-ball,  along 
the  electric  flux  in  Fig.  7,  but  there  will  be  no 
tendency  to  affect  the  direction  of  a  magnetic 
compass-needle.  If,  however,  the  electrified 
system  be  moved  bodily  sidewise,  without  losing 
the  charge,  feeble  magnetic  forces  will  be  de- 
veloped in  directions  at  right  angles  both  to  the 


28  WIRELESS  TELEGRAPHY 

moving  electric  flux  and    to    the    direction   of 
motion. 

Just  as  sidewise-moving  magnetic  flux  gener- 
ates electric  flux,  so  sidewise-moving  electric 
flux  generates  magnetic  flux. 


CHAPTER   IV 

ELECTROMAGNETIC     WAVES    GUIDED    OVER 
CONDUCTORS 

Automatic  Movement  0}  Electric  Flux  over 
Conducting  Surfaces 

IN  order  to  bring  electric  flux  into  movement, 
it  is  not  necessary  to  move  a  charged  system  of 
conductors,  the  flux  will  set  itself  in  motion  if  an 
opportunity  is  offered  to  let  it  slide  upon  a  pair 
of  conductors. 

Electric  Current  Over  a  Pair  o]  Wires 
If  we  bring  a  long  pair  of  parallel  insulated 
metallic  wires  M  N,  P  Q,  Fig.  8,  simultaneously 
into  contact,  one  with  the  rod  and  the  other  with 
the  disk,  as  indicated  in  the  figure,  the  electric 
flux  immediately  takes  advantage  of  the  exten- 
sion of  the  system  thus  offered  and  glides  away, 
guided  by  the  wires.  It  may  be  considered  that 
the  sidewise  repulsion  of  the  flux  tends  to  make 
it  spread  its  boundaries  in  this  way,  whenever 
possible.  The  electric  charge  moves  out  along 
the  wires  hand  in  hand  with  the  electric  flux,  the 
positive  charge  spreading  along  the  upper  wire 
29 


30  WIRELESS  TELEGRAPHY 

M  N,  in  Fig.  8,  and  the  negative  charge  along  the 
lower  wire  P  Q.  The  electric  flux  runs  along 
with  these  charges,  always  bridging  over  between 
the  positive  and  negative  charges.  The  phenom- 
enon of  the  movement  of  two  parallel  moving 


?»;;;* 4   i   i    i   i   »   *   *   »   4 

at  I  ii  i  4  *  i  4 


FIG.  8. — Electric    Flux   Wave   Moving    Over    Pair  of 
Parallel  Insulated  Wires. 

charges  with  the  moving  electric  flux  between 
them  and  linking  them,  constitutes  an  electric 
current,  or  electric  discharge. 

The  effect  of  bringing  the  two  parallel  wires 
into  contact  with  the  charged  electric  system  of 
Fig.  7,  is,  therefore,  to  let  the  charge  escape  over 
the  wires,  and  to  set  up  thereby  an  electric  cur- 
rent over  the  wires.  The  current  rush  takes 
place  in  the  form  of  a  wave.  Electric  flux  and 
its  associated  energy  move  off  the  disk  into  the 
insulating  air-space  between  the  wires. 

Creation  of  Magnetic  Flux  by  Moving  Electric 
Flux 

We  have  already  seen  that  sideways-moving 
electric  flux  generates  magnetic  force  and  flux. 


ELECTROMAGNETIC  WAVES  31 

As  soon,  therefore,  as  the  electric  flux  begins  to 
move,  half  of  the  electric  flux  energy  disappears 
and  is  replaced  in  the  form  of  magnetic  energy. 
Instead  of  having  stationary  electric  flux  in  the 
confined  insulated  system  of  Fig.  7,  we  have 
moving  electric  flux,  and  magnetic  flux  associated 
therewith,  or  advancing  with  it.  Consequently, 
although  we  can  have  either  stationary  electric 
flux  alone,  or  stationary  magnetic  flux  alone,  we 
cannot  preserve  them  independently  when  they 
move  freely  in  an  insulator.  Any  wave  of  elec- 
tric disturbance  is  an  electromagnetic  wave,  be- 
cause in  it  electric  and  magnetic  fluxes  are  tied 
up  together. 

Electromagnetic    Wave    Guided    by    a    Pair    o) 
Parallel  Wires 

The  distributions  of  the  electric  and  magnetic 
fluxes  in  the  advancing  wave  of  Fig.  7  are  illus- 
trated in  Fig.  9,  where  M  P  are  the  sections  of 
the  two  parallel  wires  in  a  plane  at  right  angles  to 
their  length.  The  electric  flux-paths  are  indi- 
cated, as  in  previous  instances,  by  lines  of  points 
with  arrow  heads;  while  the  magnetic  flux-paths 
are  indicated  by  broken  lines  with  arrow  heads. 
The  wave  is  supposed  to  be  receding  from  the 
observer,  and  the  upper  wire  M  is  carrying  the 
positive  charge,  while  P,  the  lower  wire,  carries 


WIRELESS  TELEGRAPHY 


the  negative  charge.  The  electric  flux,  there- 
fore, emerges  from  the  surface  of  the  wire  M  and 
terminates  upon  the  surface  of  the  wire  P.  If 
the  metal  of  which  the  wires  are  composed  be 
supposed  to  conduct  perfectly,  the  electric  flux 

will  skim  over  the  sur- 
faces of  the  wires  and 
not  penetrate  into  their 
mass.  The  more  im- 
perfect the  conductivity 
of  the  wires,  the  more 
deeply  the  moving  elec- 
tric flux  will  penetrate 
into  them. 

The  magnetic  flux  at 

netic  Fluxes  in  Electro-  the  center  O  of  the  loop 
bv  Two  Parallel  Wires,  has  the  direction  D  O 
C,  perpendicular  to  the 
loop.  At  all  other  points 
the  magnetic  flux-paths  are  circular  in  this  plane 
of  cross-section,  or  cylindrical  with  regard  to  a 
length  of  the  wires.  Both  the  electric  and  the 
magnetic  flux-paths  are  systems  of  circles,  and  it 
is  to  be  noticed  that  at  every  point  they  intersect 
each  other  perpendicularly.  That  is,  any  one 
circle  crosses  all  the  intersecting  circles  at  right 
angles. 

It  is  also  to  be  observed  that  where  the  electric 


FIG.  9. — Electric  and  Mag 


magnetic  Wave  Guided 

T\v 
'  ave    Receding 


by 
W 
Observer. 


from 


ELECTROMAGNETIC  WAVES  33 

flux  runs  densest,  so  does  the  magnetic  flux.  The 
densest  electric  and  magnetic  fluxes  are  found 
close  to  either  wire.  Both  the  fluxes  get  weaker 
as  we  recede  from  the  wires.  In  fact  the  inten- 
sity of  the  electric  flux  in  any  single  pure  electro- 
magnetic  wave  is  always  and  everywhere  numeri- 
cally equal  to  the  intensity  of  the  magnetic  flux 
at  the  same  point  and  time,  when  each  is  meas- 
ured in  its  appropriate  units.  At  an  indefinitely 
great  distance  from  the  loop  of  active  wires  the 
density  of  the  fluxes  is  nil. 

Speed  0}  Electromagnetic  Waves 

The  speed  of  sound  waves  in  air  we  have  seen 
to  be  in  the  neighborhood  of  333  meters  per 
second,  (1090  feet  per  second  or  746  miles  per 
hour).  But  the  speed  of  a  free  electric  wave  in 
air  is  enormously  greater,  being  approximately 
300,000  kilometers  per  second  (186,400  miles 
per  second),  or  7^  times  around  the  world  in  a 
second.  This  is  also  the  speed  at  which  light 
travels  in  air.  That  is  to  say,  no  difference  has 
yet  been  determined  between  the  speed  of  electro- 
magnetic waves  in  air  and  the  speed  of  light. 

Energy  is  carried  in  the  advancing  electro- 
magnetic wave  indicated  in  Figs.  8  and  9.  The 
energy  is  the  energy  residing  in  all  the  electric 
flux  that  moves  on,  plus  the  equal  amount  of 


34  WIRELESS  TELEGRAPHY 

energy  in  all  the  associated  and  interlinked 
magnetic  flux.  This  energy  is  carried  away  from 
the  original  stock  of  electric  energy  in  the  air- 
space of  the  electrified  system  in  Figs.  6  and  7. 
The  energy  was  originally  bound  up  in  the  sta- 
tionary electric  flux.  The  advancing  electric 
and  magnetic  fluxes  in  the  wave  robbed  the 
charged  system  of  flux  and  of  energy  and  trans- 
ported that  energy  whithersoever  they  went. 

Summing  up  the  conditions  which  we  have 
noted  in  the  guided  electromagnetic  wave  of  Figs. 
8  and  9  we  may  state  them  as  follows: — 

An  electric  or  magnetic  disturbance  associated 
with  a  pair  of  insulated  aerial  wires  propagates 
itself  along  the  wires  at  the  speed  of  light.  The 
wave  consists  of  electric  and  magnetic  fluxes, 
which  are  always  perpendicular  to  each  other 
and  to  the  direction  in  which  the  wave  is  moving. 
If  the  two  wires  are  parallel,  the  fluxes  are  dis- 
tributed cylindrically;  i.e.,  circularly  in  any 
plane  perpendicular  to  the  wires.  The  energy 
in  each  flux  is  the  same,  and  the  intensity  of  the 
two  fluxes  is  the  same  at  every  point.  The  energy 
per  unit  volume  varies  as  the  square  of  the  in- 
tensity of  the  moving  fluxes.  The  advancing 
wave  conveys  this  energy  with  it.  On  the  sur- 
faces of  the  wires  are  opposite  electric  charges, 
moving  with  the  flux,  and  supporting  the  ends  of 


ELECTROMAGNETIC  WAVES  35 

the  electric  flux.     The  entire  series  of  associated 
phenomena  is  an  electric  current. 

Guided  electromagnetic  waves  properly  belong 
to  the  domain  of  ordinary  wire  telegraphy,  or  to 
the  transmission  of  electric  power  by  wires.  As 
such,  they  lie  outside  of  the  province  of  this  en- 
quiry. It  may  suffice  to  observe  that  the  steady 
electric  current  found  in  any  electric  circuit  oper- 
ated by  a  dynamo,  or  a  voltaic  battery,  is  merely 
the  sum  of  what  is  usually  a  large  number  of 
superimposed  electromagnetic  waves  of  the  type 
above  considered.  These  waves  are  kept  stream- 
ing out  of  the  dynamo,  and  are  also  reflected 
back  from  the  distant  end,  or  other  parts,  of  the 
circuit;  so  that  after  a  brief  interval  of  time  we 
have  a  complex  aggregate  of  waves  present.  We 
may  now  proceed  to  the  study  of  semi-guided 
electromagnetic  waves,  or  waves  in  which  the 
electric  flux  is  held  at  one  end  only  on  an  insu- 
lated artificial  conductor,  or  is  guided  by  but  a 
single  wire. 


CHAPTER   V. 

ELECTROMAGNETIC    WAVES    GUIDED    BY    SINGLE 
WIRES 

Electromagnetic  Wave  Guided  by  a  Single  Wire 
and  the  Ground 

IF  we  place  the  disk  C  O  D  of  Fig.  7  upon  the 
level  surface  of  the  ground,  taking  pains  to 
secure  good  electrical  conductivity  in  the  adja- 
cent soil,  and  charge  the  vertical  rod  A  B,  while 
supporting  the  same  insulated  above  the  center 
O,  there  will  be  but  little  change  effected  in  the 
charge  or  distribution  of  the  electric  flux  by  rea- 
son of  the  grounding  of  the  disk.  From  an 
electrical  point  of  view,  we  shall  merely  have  in- 
definitely extended  the  area  of  the  conducting 
disk.  If  we  now  bring  a  single  very  long  insu- 
lated horizontal  wire  M  N,  like  an  ordinary  tele- 
graph wire  into  contact  with  the  charged  rod,  as 
indicated  in  Fig.  10,  the  electric  charge  and  elec- 
tric flux  will  immediately  rush  out  at  light-speed 
over  this  wire  in  an  electromagnetic  wave.  The 
electric  flux  will  be  guided  by  the  wire  M  N  on  its 
36 


ELECTROMAGNETIC  WAVES  37 

positive  ends;  but  the  negative  ends  will  be  un- 
constrained, or  left  loose  to  themselves.  In  this 
sense,  the  wave  is  only  singly  guided.  If  the 
surface  of  the  earth  G  G  be  assumed  to  conduct 
perfectly,  the  electric  flux  will  skim  over  this  sur- 
face, and  a  negative  charge  will  also  distribute 


.  -r\\i  i  *  j  4  t  i  i  1 1  1 1  I 
f*s*S>$&  iiiiiM***1* 


FIG.  10. — Single- Wire  Guided  Electromagnetic  Wave. 

itself  over  the  same,  advancing  with  the  electric 
flux. 

The  electric  flux  under  these  circumstances 
will  spread  out  over  the  surface  of  the  ground 
G  G  in  such  a  distribution  as  would  be  effected 
if  the  ground  were  removed  and  in  its  place  a 
second  wire  were  run  parallel  to  M  N  and  as  far 
beneath  the  level  surface  G  F  G  as  the  first  wire 
M  N  is  above  it.  The  distribution  is  indicated 
in  Fig.  n,  where  M  is  the  section  of  the  wire  in  a 
plane  at  right  angles  to  its  length,  and  G  G  is  the 
conducting  surface  of  the  ground.  The  wave  in 
this  case  is  supposed  to  be  advancing  towards  the 
observer.  The  lines  of  points  show  the  paths  of 
electric  flux  issuing  from  the  positive  charge 
moving  along  the  wire  M.  They  terminate  at  a 


38 


WIRELESS  TELEGRAPHY 


negative  charge  distributed  over  G  G,  and  mov- 
ing over  the  same  with  like  rapidity.  N  is  the 
position  of  the  "  image"  wire,  which,  in  the 
absence  of  the  ground  G  G,  would  be  able,  as  in 
Fig.  9,  to  produce  the  same  distribution  of  fluxes 
above  the  level  G  O  G,  as  is  developed  with  the 


FIG.  ii. — Electric  and  Magnetic  Fluxes  in  Electro- 
magnetic Wave  Guided  by  a  Single  Wire  Over  a 
Conducting  Ground  Surface.  Wave  Advancing 
Toward  Observer. 

conducting  ground.  If  the  ground  conducted 
perfectly,  the  electric  flux  would  not  penetrate 
below  the  surface;  but  would  slide  frictionless 
over  the  same.  In  practice,  the  conductivity  of 
the  ground  is  never  perfect  and  the  fluxes  pene- 
trate beneath  the  surface  to  a  greater  or  less  ex- 
tent, with  a  corresponding  expenditure  of  energy 
in  the  soil.  Nevertheless,  the  conditions  are 


ELECTROMAGNETIC  WAVES  39 

ordinarily  regarded  as  a  slight  deviation  from  the 
condition  of  perfect  earth  conduction  as  indicated 
in  the  Figure. 

The  magnetic  flux  is  established  in  cylindrical 
distribution,  as  shown,  by  the  motion  of  the 
'electric  flux  at  the  light-speed.  The  two  fluxes 
have  equal  densities  and  energies  at  any  given 
point,  and  between  them  they  transport  to  a  dis- 
tance, along  the  wire,  the  energy  originally  bound 
up  in  the  stationary  electric  flux  of  the  charged 
system  of  Fig.  7. 

The  process  thus  initiated  pertains  to  single- 
wire  telegraphy,  the  usual  type  of  wire  telegraphy. 
The  currents  employed  in  telegraphy  consist  of 
such  electromagnetic  waves,  either  singly,  or  in 
superposition  by  confluence. 


CHAPTER   VI 

RADIATED  ELECTROMAGNETIC  WAVES  GUIDED  B\ 
THE   GROUND 

Radiation  of  Electromagnetic  Waves  by  an  Electric 
Disturbance  or  Explosion 

THE  electromagnetic  waves  considered  in  the 
last  two  chapters  were  set  in  motion  by  bringing 
a  wire,  or  a  pair  of  wires,  into  electric  connection 
with  the  charged  electric  system,  and  allowing 
the  electric  flux  to  overflow  from  that  system  on 
to  and  along  the  wires.  But  electromagnetic 
waves  may  also  be  set  in  motion  by  sudden  dis- 
turbances of  an  electric  charge.  In  such  cases 
the  emitted  waves  are  likely  to  be  radiated  in  all 
directions  in  a  manner  resembling  the  expansion 
of  a  sound  wave  in  air  as  outlined  in  Chapter  II. 

Let  us  suppose  the  rod  and  disk  system  to  be 
charged,  as  indicated  in  Fig.  7,  after  the  disk  has 
been  placed  horizontally  upon  the  surface  of 
good  conducting  ground.  Instead  of  allowing  a 
wire  to  approach  the  rod  and  discharge  it,  let  the 
system  be  discharged  by  a  spark  between  A  and 
O  (Fig.  7).  Let  us  assume  that  the  discharge  is 
40 


RADIATED  ELECTROMAGNETIC  WAVES  41 

completed  by  a  single  spark  of  extremely  short 
duration;  so  that  the  entire  system  of  electric 
flux  collapses  precipitately.  The  flux  near  the 
axis  A  O  is  the  first  to  disappear  into  the  spark, 
then  the  longer  and  outlying  flux.  Last  of  all, 
the  longest  and  furthest  reaching  flux  issuing 
from  B,  will  run  down  the  rod  and  vanish  at  the 
gap  A  O. 

If  the  discharge  be  delayed,  or  the  charge 
allowed  to  dribble  slowly  across  the  gap  A  O,  as, 
for  example,  by  the  action  of  rough  and  oxydised 
opposing  surfaces,  the  collapse  of  the  entire 
umbrella-shaped  electric  flux  system  of  Fig.  7 
may  take  place  without  any  appreciable  electro- 
magnetic disturbance.  The  energy  stored  away 
in  the  flux  will  be  expended  in  heating  the  path 
of  discharge,  and,  when  the  process  is  complete, 
the  flux  having  disappeared,  the  discharge  stops, 
and  there  is  no  aftermath. 

If,  however,  the  collapse  of  the  umbrella  flux 
system  in  Fig.  7  is  sudden,  the  rapid  descent  of 
the  flux  down  the  rod  and  into  the  gap  A  O  sets 
up  magnetic  forces  and  magnetic  flux.  We  have 
already  seen  that  when  electric  flux  moves,  it 
establishes  magnetic  flux  in  a  direction  across 
itself,  and  also  across  the  direction  of  motion; 
also  that  energy  is  imparted  to  the  magnetic  flux 
at  the  expense  of  the  electric  flux  energy.  If  we 


42  WIRELESS  TELEGRAPHY 

consider,  for  example,  the  particular  flux  path 
L  M  D  in  Fig.  7,  it  is  evident  that  during  the 
brief  interval  of  collapse  it  tends  to  run  at  light- 
speed  into  the  successive  positions  H  G  K,  h  g  k, 
and  so  on  to  A  O,  when  it  disappears  into  the 
spark  discharge.  But  the  movement  of  this  flux 
element  will  set  up  magnetic  flux  directed  in 
planes  parallel  to  the  disk,  and  pointing  towards 
the  observer.  On  the  other  side  of  the  rod  A  B, 
the  similar  inrush  of  electric  flux  will  beget  a 
magnetic  flux  in  horizontal  planes,  i.e.,  planes 
parallel  to  the  disk,  but  directed  away  from  the 
observer.  Considering  all  the  actions  that  occur 
simultaneously,  it  will  be  evident  that  a  concen- 
tric ring  system  of  magnetic  flux  will  be  set  up, 
as  in  Fig.  12,  around  the  rod  A  B  as  axis,  each 
ring  being  in  a  horizontal  plane.  Part  of  the 
energy  of  the  original  electric  flux  of  Fig.  7  is 
delivered  to  this  ring  distribution  of  magnetic 
flux;  so  that  when  the  discharge  is  sudden,  a 
lesser  total  energy  tumbles  into  the  spark  than 
when  the  discharge  is  slow. 

Fig.  12  indicates,  in  plan  view,  the  ring  dis- 
tribution of  magnetic  flux  accompanying  the 
collapse  of  the  umbrella  electric  distribution  of 
Fig.  7,  on  the  passage  of  a  spark  at  O  A.  The 
eye  of  the  observer  is  supposed  to  be  situated 
immediately  over  the  disk  C  D  and  rod  B.  The 


RADIATED  ELECTROMAGNETIC  WAVES  43 

magnetic  flux  streams  are  all  directed  clockwise, 
and  they  lie  in  various  horizontal  planes. 

According  to  our  provisional  theory,  we  found 
in  Chapter  III,  at  page  25,  that  the  original 
charge  of  the  rod-and-disk  system  of  Fig.  7  gave 


FIG.  12. — Plan  View  Diagram  of  Magnetic  Flux  Distri- 
bution Accompanying  Collapse  of  Electric  Flux 
Around  Charged  Rod. 

a  right-handed  or  clockwise  screw-twist  to  the 
surrounding  ether,  as  viewed  by  an  observer 
looking  down  on  the  disk.  The  ether  may  be 
supposed  to  have  taken  a  right-handed,  or  clock- 
wise, elastic  set  or  strain,  under  the  stress  of  the 
electric  flux,  which  stress  is  supposed  to  be  re- 


44  WIRELESS  TELEGRAPHY 

sisted  by  the  elastic  rigidity  of  the  ether.  The 
stress  is  a  maximum  at  the  axis  O  A.  If  the 
ether  gives  way,  by  the  disruption  of  the  aii 
particles  at  this  point,  the  rigidity  fails  to  oppose 
the  stress,  and  the  ether  flows  clockwise  bodily 
in  the  magnetic  stream  lines  of  Fig.  12.  It 
comes  to  the  same  thing,  therefore,  whether  we 
view,  in  imagination,  the  collapsing  electric  flux 
during  the  sudden  discharge  of  the  system,  and 
watch  the  magnetic  flux  rings  spring  into  exist- 
ence  with  the  downward  electric  motion,  or 
whether  we  view  in  imagination  the  ether  give 
way  under  the  screw  twist  of  the  original  charge, 
and  watch  the  flow  of  the  ether  in  obedience  to 
that  twist  when  the  spark  occurs  at  the  axis. 

Electromagnetic   Wave  Generated   by   Sudden 
Collapse  of  Electric  Flux  Distribution 

The  ring  magnetic  flux,  as  in  Fig.  12,  accom- 
panies the  collapsing  electric  flux  down  the  rod; 
it  also  sets  up,  by  reaction,  an  external  wave  of 
upward  and  outwardly  rising  magnetic  ring  flux 
in  the  counter-clockwise  direction.  This  rising 
magnetic  flux  sets  up  in  its  turn  electric  flux 
across  itself.  The  direction  of  this  rising  shell 
of  electric  flux  is  indicated  in  Fig.  13  at  A  C  and 
A  D.  It  is  directed  from  the  rod  to  the  disk, 
as  in  the  original  charge  distribution  of  Fig.  7, 


RADIATED  ELECTROMAGNETIC  WAVES  45 

This  hemispherical  shell  of  downward  electric 
flux,  and  ring  magnetic  flux,  expands  radially 
outwards  in  all  directions  at  the  light-speed. 

The  collapsing  ring  magnetic  flux  of  Fig.  12, 
when  it  reaches  the  disk,  is  reflected  back  and  up 


FIG.  13. — Electric  Flux  Induced  by  the  Ring  Distribu- 
tion of  Magnetic  Flux  in  Fig.  12. 

the  rod,  still  clockwise  in  direction,  but  moving 
upwards  immediately  behind  the  shell  C  B  D. 
It  sets  up  an  electric  flux  in  the  directions  indi- 
cated at  A,  Fig.  13.  The  two  concentric  hemi- 
spheres of  electric  and  magnetic  fluxes  detach 
themselves  from  the  rod  in  the  manner  dia- 
grammatically  indicated  in  Fig.  14.  In  the 
external  hemispherical  shell  w  P  w,  the  electric 
flux  is  downwards  and  the  magnetic  flux  lies  in 
counter-clockwise  rings  centered  on  the  polar 
axis  B  P.  In  the  internal  hemispherical  shell 
x  Q  x,  the  directions  are  reversed,  the  electric 
flux  being  upwards,  and  the  magnetic  clockwise. 
In  a  very  brief  interval  of  time  after  the  dis- 
charge of  the  system  by  the  passage  of  the  spark, 
we  have  complete  disappearance  of  electro- 
magnetic charges,  fluxes  and  energy  in  the  rod- 


46  WIRELESS  TELEGRAPHY 

jnd-disk  system,  while  a  hemispherical  electro- 
magnetic wave  moves  off  radially  with  the  light- 
speed,  the  radius  of  the  hemisphere  being  theoret- 
ically 300,000  kilometers  (186,400  miles)  after 
one  second  of  time.  The  thickness  of  the 


*//.£/ !«V  ? 


.*••"£•»» 


*  V, 

*  '<*\ 

%    *  I* 

'.  '.'• 


FIG.   14. — Vertical  Cross-Section  and  Plan  of  Single 
Expanding  Electromagnetic  Wave. 

double-layered  hemispherical  shell  remains  con- 
stant, but  since  the  energy  in  the  shell  also  re- 
mains constant,  in  the  absence  of  absorption,  the 
density  of  the  fluxes  and  their  energy  per  unit  of 
volume  rapidly  diminish.  In  other  words  the 
energy  per  cubic  meter  of  space  in  the  wave 
rapidly  diminishes. 


RADIATED  ELECTROMAGNETIC  WAVES  47 

The  feet  of  the  electric  flux  lines  skim  over  the 
surface  of  the  ground,  assumed  to  be  perfectly 
conducting.  At  the  external  edge  w  w  w  w,  a 
negative  ring  charge  runs  out  radially  over  the 
ground  surface  at  the  light- speed.  At  x  x,  a 
similar  positive  ring  charge  runs  out  radially  at 
the  same  speed.  This  succession  of  running 
electric  charges,  linked  together  by  loops  of  elec- 
tric flux,  constitutes  a  single  cycle  0}  alternating 
current  flowing  along  the  ground. 

From  its  external  aspect,  the  expanding  hemi- 
spherical electromagnetic  wave  has  electric  flux 
distributed  along  meridians  of  longitude,  sym- 
metrically disposed  with  respect  to  the  polar  axis 
B  Q  P.  The  magnetic  flux  is  distributed  in 
circles  of  latitude,  the  smallest  circles  being  near 
the  pole,  and  the  greatest  near  the  equator  or 
ground  surface. 

Resemblances  Between  Solitary  Explosion  Waves 
of  Sound  and  Electromagnetism 

The  solitary  hemispherical  electromagnetic 
wave  of  Fig.  14  bears  some  resemblance  to  the 
solitary  hemispherical  sound  wave  in  air  of  Fig.  3. 
Each  consists  of  a  double  layer,  the  disturbance 
in  the  external  layer  being  of  the  opposite  sign  to 
that  in  the  internal  layer.  On  the  other  hand, 
there  are  notable  differences.  There  is  an 


48  WIRELESS  TELEGRAPHY 

enormous  difference  in  speed  (nearly  a  million 
to  one).  The  electric  wave  has  a  polar  node  at 
P  Q  and  the  sound  wave  has  none.  The  electric 
wave  is  propagated  in  the  ether,  the  sound  wave 
in  a  gaseous  substance. 

If  the  earth's  surface  is  not  perfectly  conduct- 
ing, and  in  practice  it  is  far  from  being  perfectly 
conducting,  the  electric  flux  will  penetrate  to 
some  extent  into  the  soil,  carrying  also  magnetic 
flux  with  it.  The  fluxes  which  sink  in  this  way 
expend  their  energy  in  warming  the  soil  very 
slightly  and  the  hemispherical  wave  is  thereby 
drained  of  a  part  of  its  energy,  or  is  subjected  to 
frictional  losses  in  running  along  the  ground. 
The  energy  per  cubic  meter  of  wave  shell  will 
thus  diminish  more  rapidly  than  would  be  ac- 
counted for  by  the  mere  increase  in  bulk  of  the 
expanding  wave  shell.  • 

Electromagnetic  Wave-trains 

The  discharge  of  a  rod-and-disk  electrified 
system  does  not  ordinarily  give  rise  to  but  a 
single  electromagnetic  wave,  such  as  is  depicted 
in  Fig.  14.  On  the  contrary,  the  discharge  gen- 
erally gives  rise  to  a  series,  or  train,  of  successive 
waves  of  diminishing  amplitude,  each  feebler 
than  its  predecessor.  The  rate  of  diminution 
depends  upon  the  amount  of  heat  expended  in 


RADIATED  ELECTROMAGNETIC  WAVES  49 

the  discharge  spark,  and  to  a  lesser  extent  upon 
other  structural  details,  but  the  amplitude  of 
oscillation  usually  falls  to  one  half,  or  loses  fifty 
per  cent.,  in  about  two  complete  swings,  or 
after  two  successive  waves  have  been  thrown  off. 
If  the  spark  remained  uniform,  the  amplitude 
would  in  such  a  case  fall  to  one  half  again  in  two 
more  swings,  or  to  one  quarter  of  the  original 
amplitude  in  a  total  of  four  swings,  or  to  one 
eighth  in  six  swings  and  so  on.  Consequently, 
the  amplitude  of  the  successive  waves  emitted  by 
a  simple  rod  oscillator  soon  dwindles  into  in- 
significance. 

Analysis  of  Oscillatory  Current  on  Rod  and  the 
Generation  of  Waves 

It  may  be  of  interest  to  consider  in  some  detail 
the  process  of  emitting  a  train  of  hemispherical 
electromagnetic  waves  from  a  rod-and-disk  sys- 
tem laid  on  a  perfectly  conducting  ground  and 
suddenly  set  into  electric  oscillation  by  a  spark 
discharge.  Such  a  system  may  be  briefly  de- 
scribed as  a  simple  vertical  oscillator.  Reference 
is  made  to  Fig.  15,  where  the  vertical  rod  is  repre- 
sented in  nine  successive  stages  of  the  process  of 
manufacturing  and  shipping  half  of  an  electro- 
magnetic wave,  and  two  such  diagrams  would  be 


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50 


RADIATED  ELECTROMAGNETIC  WAVES   51 

required  to  illustrate  the  delivery  of  one  com- 
plete magnetic  wave  into  free  space.  Instead  of 
commencing  with  the  insulated  rod  just  prior  to 
the  spark  discharge,  as  in  Fig.  7,  it  is  more  con- 
venient to  commence  with  the  condition  indi- 
cated at  A,  Fig.  15,  where  the  electric  flux  has 
just  completed  its  movement  at  light-speed  up  to 
the  top  of  the  rod,  or  has  climbed  up  to  the  sum- 
mit of  the  conductor.  The  flux  arrows  touching 
the  rod  are  pointing  inwards,  indicating  a  nega- 
tive charge  on  the  surface  of  the  rod.  Ascending 
electric  flux  is  marked  by  solid  arrows  and  de- 
scending flux  by  broken  or  dotted  arrows. 

Accompanying  the  upward  movement  of  the 
converging  flux  which  has  culminated  in  the  con- 
dition at  A,  there  will  be,  as  previously  stated,  an 
associated  magnetic  flux.  The  direction  of  this 
magnetic  flux  is  indicated  by  the  device  of  a  small 
circle  and  a  small  upright  bar,  the  former  on  the 
left-hand  of  the  rod,  and  the  latter  on  the  right 
hand.  The  circle  may  be  looked  upon  as  the 
feathers,  or  heel,  and  the  small  bar  as  the  point, 
or  barb,  of  an  arrow  in  a  horizontal  plane  bent 
into  a  semicircle  about  the  rod  on  the  side  remote 
from  the  observer.  The  device  is  illustrated  in 
detail  in  Fig.  i5a,  where  P  Q  is  a  vertical  section, 
and  p  q  a  plan  view,  of  a  converging  plane  of 
electric  flux  terminating  on  the  rod  E  F  at  its 


WIRELESS  TELEGRAPHY 


FIG.  i5a. — Sectional  Ele- 
vation and  Plan  of  *a 
Plane  Electromagnetic 


center  and  sliding  up- 
wards from  E  to  F  over 
the  rod's  surface,  like  a 
ring  over  a  peg.  When, 
as  shown  in  Fig.  1 5a,  the 
small  circle  c  is  on  the 
left  hand,  and  the  bar  b 
on  the  right  hand,  the 
ring  magnetic  flux  cf  d  b' 
is  disposed  clockwise 
about  the  rod  as  viewed 

from    above.      Counter 

clock-wise  movement  of 

Directions    of    Fluxes,     magnetic    flux    calls   for 
Key  Diagram  to  Fig.  15.  .  .  , 

the  circle  on  the   right 

hand  and  the  bar  on  the  left. 

Rules  for  Memorizing  the  Directions  oj  Motion, 
and  oj  Electric  Flux  and  oj  Magnetic  Flux 
in  Any  Single  Free  Electromagnetic  Wave. 

There  is  a  simple  law  connecting  the  directions 
of  electric  and  magnetic  fluxes  in  any  simple 
plane  electromagnetic  wave.  It  may  be  ex- 
pressed mnemonically  in  either  of  the  following 
ways: 

(i)  Draw  an  arrow  in  the  direction  in  which 
the  electric  flux  points.  Let  the  head  of  the 
arrow  be  supposed  to  be  the  head  of  a  man  who 


RADIATED  ELECTROMAGNETIC  WAVES  53 

runs  in  the  direction  in  which  the  flux  is  running. 
Then  the  man's  side-extended  right  hand  will 
point  in  the  direction  of  the  magnetic  flux.  This 
rule  is  illustrated  mnemonically  in  Fig.  16.  The 


FIG.  16. — Memory  Picture  for  Recalling  Directions  of 
Electric  and  Magnetic  Fluxes  in  Waves. 

point  of  the  Greek  warrior's  sword  is  supposed 
to  be  magnetized  and  supporting  iron  nails. 

(2)  When  electric  flux,  converging  upon  a  con- 
ducting rod  or  cylinder,  as  in  Fig.  15  a,  points  its 
arrows  inwards  like  the  V's  in  the  face  of  a  clock 
(See  Fig.  17),  then  if  the  flux  is  moving — like  the 
light  that  makes  them  visible — from  the  clock 
towards  the  observer,  the  magnetic  flux  M  will 
point  circularly  in  the  direction  of  the  Motion  of 


54 


WIRELESS  TELEGRAPHY 


the  clock  hands.  Reversing  either  the  electric 
arrows  E,  E,  E,  or  their  movement  towards  the 
observer,  reverses  the  direction  of  the  magnetic 
flux;  but  reversing  both,  leaves  the  magnetic 
flux  M  pointing  clockwise. 

Release  of  Electromagnetic   Wave  from  Simple 
Rod  Oscillator 

Applying  either  of  the  rules  to  the  upwardly 
moving  and  inwardly  pointing  horizontal  electric 

flux  on  the  rod  at  A,  Fig. 
15,  it  will  be  evident 
that  the  magnetic  flux 
is  directed  clockwise,  as 
viewed  from  above. 

The  moment  the  elec- 
tric flux  reaches  the  top 
of  the  rod  it  is  reflected 
Re- 


FIG.  17. — Mnemonic  Clock 

Diagram  of  Relative    from  the  free  end. 


flection  of  electric  flux 


Directions    of    Electric 
Flux  E   and    Magnetic 

Flux  M  in  an  Electro-    from  a  free  end  always 

the    direction 


magnetic  Wave  Coming 
Towards   the   Observer 


requires 

of  the  magnetic  flux  to 
be  reversed,  leaving  the 
electric  flux  arrows  unchanged  in  direction,  but 
moving  backwards,  or  retreating.  This  relation 
is  a  consequence  of  either  of  the  above  rules. 
Referring  to  Fig.  15  a,  the  flux  which  has  reached 


RADIATED  ELECTROMAGNETIC  WAVES  5^ 

the  top  of  the  rod  at  light- speed  must  keep  on 
moving,  and  since  it  can  go  no  further  up- 
wards, it  commences  to  descend  at  light-speed. 
In  Fig.  15,  descending  electric  flux  is  shown  by 
dotted  line  arrows  and  ascending  flux  by  heavy 
line  arrows. 

The  reversal  of  the  direction  of  motion  of 
magnetic  flux  around  the  rod,  in  changing  from 
going  up  to  coming  down,  delivers  a  blow,  by 
inertia,  to  the  surrounding  external  ether.  In 
other  words,  the  jerk  required  to  reverse  the 
magnetic  flux  around  the  rod  between  stages  A 
and  E  of  Fig.  15,  sets  up  a  counter- jerk  or  kick 
in  the  surrounding  ether.  The  kick  sets  up  two 
free  waves  traveling  in  opposite  directions.  The 
electric  fluxes  in  these  two  waves  are  mutually 
opposite ;  but  the  magnetic  fluxes  conspire  clock- 
wise. These  relations  are  indicated  by  the  ex- 
ternal pairs  of  arrows  at  A,  which  start  into 
existence  at  the  instant  of  reflection  of  the  central 
wave  from  the  top  of  the  rod. 

At  the  instant  of  time  represented  at  B,  the 
second  stage,  there  has  been  a  movement  of  the 
flux  gliding  over  the  rod,  and  also  a  movement  in 
each  of  the  two  external  free  waves.  Taking 
these  in  order,  the  gliding  flux  has  commenced  to 
move  down  at  the  top,  or  to  double  back  upon 
itself,  the  three  lower  layers  still  climbing,  but  the 


56  WIRELESS  TELEGRAPHY 

leading  layer  descending.  The  electric  flux  ar- 
rows point  inwards  at  all  parts  of  the  rod,  but  the 
magnetic  fluxes  are  opposed  at  the  top,  or  tend 
to  neutralize  there.  In  the  external  waves,  the 
ascending  one  has  advanced  one  stage,  and  its 
wave-front  is  at  a.  The  descending  one  has 
reached  the  conducting  disk,  or  ground,  at  the 
base,  and  has  been  reflected  from  this  surface. 
Reflection  at  a  normal  conducting  surface  en- 
tails reversal  of  the  electric  flux  arrows  but  no 
reversal  of  the  magnetic  flux  arrows.  Conse- 
quently the  lowest  layer  of  the  free  external  de- 
scending wave  at  A  has  turned  its  arrows  from 
outwards  to  inwards,  and  the  ring  is  moving 
bodily  upwards. 

At  the  next  stage,  indicated  at  C,  the  leading 
half  of  the  wave  on  the  rod  has  doubled  back  on 
the  following  half,  the  electric  flux  all  pointing 
inwards,  and  the  magnetic  fluxes  completely 
neutralizing.  There  is  no  resultant  magnetic 
flux  at  this  instant  around  the  rod.  In  the  free 
external  waves,  the  ascending  front  has  reached 
b.  Half  of  the  external  wave  which  commenced 
moving  downwards  at  A,  has  doubled  back  upon 
itself  and  is  ascending. 

Continuing  this  process  to  E,  we  find  that  the 
ekctric  flux  slipping  on  the  rod  is  now  all  moving 
downwards  and  the  magnetic  flux  is  all  counter 


RADIATED  ELECTROMAGNETIC  WAVES  57 

clockwise.  This  means  in  ordinary  language 
that  the  electric  current  in  the  rod  is  at  this  in- 
stant a  maximum  in  the  downward  direction. 
The  electric  flux  arrows  all  point  inwards,  so  that 
there  is  a  negative  electric  charge  all  over  the  rod. 
In  the  external  ether,  the  direction  of  motion  is 
altogether  upward,  with  the  electric  flux  inward 
and  the  magnetic  flux  clockwise.  The  front  of 
the  emitted  wave  has  now  reached  d. 

A  new  spark  will  now  cross  the  air-gap  at  the 
base  of  the  rod,  not  shown  in  the  Figure,  and  the 
electric  flux  will  pass  over  the  conducting  spark 
column  to  the  ground  at  the  base.  It  is  reflected 
back  from  there  with  reversal  of  electric  flux  ar- 
rows, but  persistence  of  magnetic  flux;  so  that 
there  is  no  kick  or  disturbance  generated  this 
time  in  external  space.  At  F,  the  head  of  the 
wave  conducted  to  the  ground  over  the  rod  has 
turned  around  and  is  ascending.  The  external 
free  wave  has  reached  e  and  is  clear  of  the 
ground. 

At  G,  there  is  complete  neutralization  of  elec- 
tric fluxes,  there  being  no  resultant  charge  on  the 
rod  at  this  instant.  But  the  current  wave,  as 
gauged  by  the  conspiring  magnetic  flux,  is  at  its 
maximum,  and  directed  upwards. 

In  the  last  stage,  at  I,  the  flux  gliding  along 
the  rod  has  reached  its  full  development  in  the 


58  WIRELESS  TELEGRAPH^ 

upward  direction,  and  is  about  to  be  reflected 
back  from  the  free  end  at  the  top.  This  will  in- 
volve a  reversal  of  magnetic  flux  and  a  new  shock 
to  the  surrounding  ether,  but  in  the  opposite  di- 
rection to  the  shock  delivered  at  A.  A  new  pair 
of  oppositely  moving  external  waves  is  thereby 
created,  as  indicated  at  I. 

After  eight  more  stages  have  been  passed,  the 
external  wave  will  have  completely  deployed,  and 
the  length  of  this  emitted  wave  will  be  just  four 
times  the  length  of  the  rod.  The  half  wave  con- 
tained between  h  and  i,  at  I,  is  twice  the  length  of 
the  rod. 

Reviewing  the  various  stages,  it  will  be  evident 
that  the  electric  flux  reaches  its  maximum  result- 
ant value  near  the  top  of  the  rod;  while  the 
magnetic  fluxes,  on  the  contrary,  are  always  in 
opposition  at  reflections  from  the  top  and  reach 
their  maximum  near  the  bottom.  This  is  an- 
other way  of  saying  that  the  electric  charge,  and 
electric  'voltage  or  potential,  develop  maximum 
amplitude  in  oscillation  at  the  top  of  the  rod,  and 
the  electric  current  at  the  base. 

The  diagram  of  Fig.  1 5  must  not  be  interpreted 
too  literally.  The  actual  flux  distributions  are 
somewhat  more  complex,  and  the  radius  of  the 
emitted  wave  at  I,  say,  is  not  exactly  equal  in  all 
directions  to  the  height  h  I.  The  emitted  wave 


RADIATED  ELECTROMAGNETIC  WAVES  59 

becomes  sensibly  hemispherical,  however,  after 
the  radius  has  acquired  the  length  of  one-half 
wave.  It  is  sufficient  to  observe  that  there  are 
two  sparks  for  each  complete  electromagnetic 
oscillation,  and  one  complete  oscillation  of  elec- 
tric pressure  and  current  on  the  rod  is  accom- 
panied by  the  emission  of  one  complete  free 


FK>.  1 8. — Train  of  Seven  Hemispherical  Electromag- 
netic Waves  of  Decaying  Amplitude  Liberated  by 
a  Rod  Oscillator  at  Center. 

hemispherical  wave  into  space.  The  energy 
contained  in  the  fluxes  of  the  wave  are  drawn 
from  the  energy  of  the  fluxes  oscillating  up  and 
down  the  rod,  which  are  thereby  constantly  being 
weakened,  and  reduced  in  density. 

A  diagrammatic  vertical  cross- section  of  seven 
complete  hemispherical  waves  is  seen  in  Fig.  18, 
as  emitted  from  a  simple  rod  and  grounded  disk 
oscillator  o  at  the  center.  The  first  wave  has 
attained  the  radius  o  w,  about  28  rod-lengths 
from  the  center;  while  the  seventh  wave  has  just 


60  WIRELESS  TELEGRAPHY 

been  released.  The  first  wave  was  the  most 
powerful  and  is  represented  in  the  heaviest  lines. 
Each  successive  wave  is  weaker  and  weaker.  It 
should  be  remembered,  however,  that  the  heavi- 
ness of  the  lines  in  the  illustration  only  relates  to 
the  strength  of  each  wave  at  the  moment  of  its 
release,  or  at  the  moment  when  it  passes  a  given 
point;  for  as  each  wave  expands,  its  energy  per 
cubic  meter  or  cubic  foot  rapidly  diminishes,  be- 
cause the  volume  occupied  by  the  wave  rapidly 
increases.  Consequently,  the  energy  per  cubic 
meter  in  the  first  and  strongest  wave,  by  the  time 
it  has  reached  the  position  w  x  w,  may  be  much 
less  than  the  energy  per  cubic  meter  in  the  shell 
of  the  last  and  feeblest,  but  most  condensed,  wave 
at  the  center,  O. 

At  any  point,  R,  the  advance  of  the  wave  is  in 
the  radial  direction  O  R,  at  the  speed  of  light. 

The  directions  of  electric  and  magnetic  fluxes 
in  this  train  of  waves  is  indicated  by  the  devices 
already  used  in  Figs.  15  and 


Deviation  oj  Electromagnetic  Waves  from  the 
Hemispherical  Form  Owing  to  the  Curva- 
ture oj  the  Earth 

According  to  the  theory  above  outlined,  the 
hemispherical  waves  of  Fig.  18  would  travel  over 
a  perfectly  flat  conducting  ground  surface  at 


RADIATED  ELECTROMAGNETIC  WAVES  6l 

light- speed  and  the  polar  radius  o  x  would  be 
300,000  kilometers  (186,400  miles)  long  after  one 
second.  In  practice,  when  such  waves  are 
thrown  out  by  a  rod  oscillator  we  have  to  deal 
with  a  moderately  conducting  spheroidal  world 
surface.  This  changes  the  shape  of  the  waves 
and  makes  them  less  geometrically  simple.  The 
waves  will  conform  to  the  curvature  of  the  earth 
and  sea,  the  successive  rings  of  positive  and 
negative  charge  running  out  in  all  directions  over 
the  earth's  surface  at  light-speed,  and  the  feet  of 
the  electric  flux  shells  gliding  along  with  them. 
The  waves  continue  to  weaken  in  intensity  per 
unit  of  volume  as  they  run,  both  on  account  of 
expanding  volume,  and  owing  to  sinking  into  the 
imperfectly  conducting  earth  surface  at  their  feet, 
i.e.,  by  frictional  dissipation  of  energy  in  the 
ground. 

Condensation  of  Fluxes,  and  of  Their  Energy, 
Towards  the  Equatorial  Zone 

Although  the  direction  of  movement  of  these 
waves  is  always  radially  outwards,  or  perpendicu- 
lar to  the  wave  front,  yet  the  density  of  the  elec- 
tric flux  in  each  wave  is  not  uniform  all  over  the 
surface  of  the  hemisphere.  It  is  greatest  near  to 
the  ground,  and  least  near  to  the  pole.  This  is 
roughly  indicated  in  Fig.  14.  Some  of  the  up- 


62  WIRELESS  TELEGRAPHY 

ward  electric  flux  in  the  inner  hemispherical  shell 
turns  back  to  the  earth  a  short  distance  above 
the  surface.  The  higher  we  rise  in  the  shell,  the 
less  flux  we  find  in  it,  and  when  we  reach  the 
polar  axis  Q  P  all  the  flux  has  ceased.  The  same 
condition  necessarily  attaches  to  the  associated 
magnetic  flux  in  the  wave.  The  result  is  that 
the  energy  contained  in  unit  volume  of  either  or 
of  both  fluxes  near  the  ground  is  greater  than  it 
would  be  according  to  simple  uniform  distribu- 
tion, by  about  60  per  cent.  In  other  words,  the 
flux  densities  and  energy  in  any  hemispherical 
electromagnetic  wave  are  greatest  at  the  ground 
or  equator  and  dwindle  towards  the  pole. 

Relations  Between  Wave-length,  Frequency,  and 
Periodic  Time 

From  the  relation  that  the  length  of  the  waves 
emitted  by  a  simple  vertical  rod  oscillator  is  four 
times  the  length  of  the  rod,  we  can  readily  find 
the  duration  of  each  wave  as  it  passes  any  point 
on  the  ground  or  in  the  air  above.  Let  us  sup- 
pose that  the  oscillator  is  kept  supplied  with 
electric  energy  so  as  to  keep  sending  out  waves 
for  just  one  second  of  time,  and  that  the  length 
of  the  rod  is,  say,  25  meters  (27.34  yards).  Then 
the  length  of  each  wave  would  be  100  meters 
(109.36  yards)  measured  along  any  radius.  But 


RADIATED  ELECTROMAGNETIC  WAVES  63 

in  one  second  of  time,  the  radius  of  the  outermost 
wave  would  have  reached  to  300,000  kilometers 
(186,400  miles)  and  this  distance  would  cover 
3,000,000  such  wave-lengths.  It  is  clear  then 
that  the  oscillator  must  have  emitted  three  mil- 
lions of  waves  in  the  one  second  of  time  con- 
sidered, and  also  that  the  time  occupied  by  the 
wave  to  pass  a  given  point  would  be 


of  a  second.  This  is  stated  in  the  customary 
phraseology  by  saying  that  the  frequency  of  oscil- 
lation is  3,000,000  cycles  per  second  and  that  the 
periodic  time  of  such  waves  is  &o'ootinnr  second. 
In  a  similar  manner,  if  we  know  any  one  of  the 
three  quantities  wave-length,  frequency,  or 
periodic-time  of  an  electromagnetic  wave  in  air, 
we  can  instantly  assign  the  other  two,  because 
the  velocity  of  propagation  is  the  light-  speed  in 
air  of  300,000  kilometers  (186,400  miles)  per 
second.  It  is  believed  that  there  is  hardly  any 
difference  between  the  speed  of  light  in  air  and 
in  free  ether  space  devoid  of  air. 


CHAPTER   VII 

UNGUIDED,  OR  SPHERICALLY  RADIATED  ELECTRO- 
MAGNETIC  WAVES 

Generation  of  Spherical  Electromagnetic  Waves 
by  the  Discharge  oj  a  Double-rod  Oscillator 

IF  we  take  a  pair  of  conducting  rods  A  B,  C  D, 
and  suitably  support  them  insulated  in  line  with 
each  other  as  indicated  in  Fig.  19,  then  on  charg- 
ing the  system  electrically,  with  the  rod  A  B,  say, 
positive,  the  electric  flux  lines  will  permeate  all 
the  surrounding  air  in  the  distribution  roughly 
depicted.  So  long  as  the  insulation  is  maintained 
there  will  be  no  magnetic  flux.  If,  however,  we 
raise  the  electrification  to  a  point  at  which  the  air 
breaks  down  between  the  two  opposed  extremi- 
ties B  C,  the  electric  flux  system  collapses  and 
runs  in  towards  the  spark.  At  the  same  time 
magnetic  flux  is  generated  in  rings  around  the 
axis  A  D  by  the  inrushing  electric  flux.  The 
sudden  generation  of  magnetic  flux  gives  a 
shock  to  the  surrounding  ether  which  sends  off 
a  spherical  electromagnetic  wave  into  surrounding 
space  at  light-speed. 

64 


UNGUIDED  ELECTROMAGNETIC  WAVES  65 

The  contours  of  five  successive  spherical  waves 
is  given  diagrammatically  in  Fig.  20,  with  refer- 
ence to  the  pair  of  discharging  rods  at  o,  the  cen- 
ter of  disturbance.  If  the  two  electrified  rods 

»' 

*         >• 

*&•" 

^    4*  *•     ••     •»•>  ~»  •+   ••»  ... 


*'    .     *   /  .  *  rfr^»   »    v    N    *    »    »    * 

J        '        -•     :    "    N-    ^    •  \    I    » 

/  «'  i  }fA*£\\  **•'*' 

*  ;  *  *  i.'mfc\*  •*  *  ;  *  * 


FIG    19. — Electric  Flux  Between  Oppositely  Charged 
Conducting  Rods. 

have  no  source  of  energy  to  maintain  their  oscil- 
lations except  the  original  charge,  the  successive 
outgoing  waves  carry  that  energy  away,  while 
some  of  the  remainder  is  dissipated  in  heat  in  the 
spark  and  also  in  the  surfaces  of  the  rods,  so  that 
the  oscillations  rapidly  die  away.  It  is  not  im- 
possible, however,  to  supply  electric  energy  to 


66 


WIRELESS  TELEGRAPHY 


the  rods  as  fast  at  it  is  radiated  externally  and 
dissipated  locally,  so  as  to  maintain  the  oscilla- 
tions indefinitely,  although  it  is  very  difficult  to 
do  this  experimentally.  In  such  a  case,  the  wave 


FIG.  20. — Diagram  of  Section  through  the  Polar  Axis 
of  a  Train  of  Five  Spherical  Electromagnetic 
Waves  Emitted  by  a  Double-Rod  Oscillator  at  the 
Center. 


train  would  go  on  extending  and  expanding  in- 
definitely at  light-speed.  If  the  rods  could  some- 
how be  placed  jn  free  space,  remote  from  the 
earth  and  all  conductors,  the  spherical  waves 
would  keep  on  moving  radially  outwards.  In 
practice,  on  this  earth,  the  waves  must  almost 
immediately  strike  the  surface  of  the  ground  on 
one  side  at  least,  and  be  reflected  there,  not  to 


UNGUIDED  ELECTROMAGNETIC  WAVES  67 

speak  of  the  influence  of  neighboring  walls,  trees, 
etc.,  so  that  the  pure  spherical  form  cannot  be 
maintained. 

Figure  20  shows  that  the  polar  axis  P  P  is  in  the 
line  of  the  rods  at  the  center  o.  On  this  axis  the 
electromagnetic  fluxes  and  energies  disappear.  At 
and  near  the  equator  Q  Q,  the  fluxes  are  densest 
and  their  energies  are  a  maximum,  for  any  given 
radial  distance  from  the  center.  The  length  of 
each  wave  is  four  times  the  length  of  either  rod, 
as  in  the  hemispherical  waves  considered  in  the 
last  chapter;  or  it  is  twice  the  length  of  the 
double- rod  oscillator  A  D  of  Fig.  19. 


Spherical  Electromagnetic  Waves  Identical  with 
Long-wave  Polarized  Light 

Physicists  are  now  agreed  that  such  an  oscilla- 
tor as  above  described,  if  kept  supplied  with 
energy  for  radiating  electromagnetic  waves, 
would  emit  light.  That  is  to  say,  such  electro- 
magnetic waves  constitute  light;  although  not 
ordinary  light  such  as  is  recognized  by  the  eye. 
The  main  difference  between  such  waves  and 
ordinary  light  lies  in  the  wave-length.  The 
human  eye  is  able  to  recognize  as  light  electro- 
magnetic waves  whose  length  lies  between  the 


68  WIFELESS  TELEGRAPHY 


limits  of  0.4  micron*  (-gu^nr  inch),  in  violet  light, 
and  0.8  micron  (-^.TOT  inch)  in  red  light. 
Electromagnetic  waves  which  are  either  shorter 
or  longer  than  this  are  not  directly  visible;  al- 
though they  may  still  be  objectively  regarded  as 
light. 

Modern   Electric    Theory    That   All   Matter   is 
Ultimately  Electricity 

Ordinary  matter,  such  as  a  piece  of  match- 
wood, is  believed  to  be  made  up  of  ultimate  par- 
ticles called  molecules,  too  small  to  be  seen  by  the 
microscope.  Molecules  are  chemical  combina- 
tions or  chemical  groups  of  atoms.  Atoms  are 
the  supposed  ultimate  particles  of  elementary 
substances,  or  the  smallest  pieces  of  such  elemen- 
tary substances  which  can  exist  separately  as 
such.  Atoms  in  their  turn  are  now  supposed  to 
be  each  constructed  of  much  more  minute  or 
ultra-ultra-microscopic  electrical  charges,  called 
electrons,  there  being  a  definite  number  and  or- 
ganization of  electrons  to  each  atom  of  an  ele- 
mentary chemical  substance.  Consequently,  all 
the  matter  in  the  universe  is  ultimately  con- 
structed, according  to  this  theory,  of  definitely 

*  The  micron  is  the  term  used  in  microscopy  for  the 
one-millionth  part  of  one  meter  from  the  Greek  mikros, 
small.  It  is  usually  designated  by  the  Greek  letter 


UNGUIDED  ELECTROMAGNETIC  WAVES  69 

organized  electric  charges,  or  of  electricity.  An 
atom  of  hydrogen,  for  example,  is  supposed  to 
comprise  about  800  electrons  in  some  definite 
organized  orbital  or  planetary  movements,  an 
atom  of  oxygen  about  10,000  electrons  in  a  dif- 
ferent grouping  of  orbits ;  and  so  on,  for  other 
elementary  substances. 

When  atoms  are  heated,  as  for  example,  the 
hydrogen  and  carbon  atoms  of  wood,  by  setting 
fire  to  a  match,  the  electric  charges  or  electrons 
within  the  atoms  are  regarded  as  being  forced 
into  rapid  and  violent  oscillation,  whereby  elec- 
tromagnetic waves  are  radiated  off.  Since  these 
atomic  oscillators  are  of  ultra-microscopic  di- 
mensions, so  too  are  the  lengths  of  some  of  their 
electromagnetic  waves.  Those  waves  whose 
lengths  lie  between  0.4  and  0.8  micron  are  per- 
ceived by  our  eyes  as  light.  A  pair  of  little  rod 
oscillators,  as  in  Fig.  19,  each  about  0.2  micron 
l°ng  (12  0*0  60  mch)  excited  into  sustained  radia- 
tion, would  give  off  waves  of  red  light,  the  longest 
waves  by  which  the  retina  of  the  eye  is  affected. 

Virtual  Ultra-microscopic  Oscillators  in  Heated 
Matter  and  Their  Emitted  Waves 

Looked  at  in  another  way,  the  shortest  electro- 
magnetic waves  that  have  yet  been  produced  bv 


70  WIRELESS  TELEGRAPHY 

the  discharge  of  electric  rods  or  spheres  are  a 
few  centimeters,  or  inches,  in  length.  In  order 
to  make  visible  light  in  the  same  manner,  we 
should  have  to  use  ultra-microscopic  particles  as 
discharging  bodies.  On  the  other  hand,  the 
waves  employed  in  wireless  telegraphy  usually 
vary  between  100  meters  (109.4  yards)  and 
10,000  meters  (6.21  miles)  in  length.  The  latter 
would  include  in  one  wave  length  25,000,000,000 
waves  of  violet  light,  the  shortest  detected  by  the 
human  eye. 

The  velocities  of  all  electromagnetic  waves  be- 
ing apparently  the  same,  whatever  .their  length, 
their  frequencies  differ  in  a  similar  range.  The 
frequency  of  a  lo-kilometer  (6.21  miles)  wave 
would  be  30,000  cycles  per  second,  or  each  oscil- 
lation would  occupy  3^  0  0  second  in  execution. 
But  the  frequency  of  violet  light  is  760  millions 
of  millions  per  second.  Each  color  of  the  spec- 
trum has  its  own  frequency  and  corresponding 
wave-length. 

There  is  one  other  difference  between  visible 
light  and  spherical  electromagnetic  waves  pro- 
duced by  electric  discharge  between  conductors 
as  in  Fig.  19.  This  is  in  regard  to  the  directions 
of  the  poles  of  the  waves.  In  Fig.  20,  the  polar 
axis  of  the  waves  always  lies  in  the  line  of  the  rods, 
no  matter  how  far  the  waves  may  extend  into 


UNGUIDED  ELECTROMAGNETIC  WAVES  71 

space.  There  is  no  energy  emitted  along  this 
axis.  Let  us  suppose  that  the  rods  are  ultra- 
microscopic,  so  that  they  are  enabled  to  emit 
waves  short  enough  to  affect  the  eye,  and  that 
their  energy  of  oscillation  is  somehow  sustained. 
Then  the  point  o  in  Fig.  20  would  be  a  luminous 
point,  shining  with  one  particular  color  of  the 
spectrum.  The  brightness  of  the  point  would, 
however,  be  greatest  in  the  equatorial  plane  Q  Q, 
and  it  would  dwindle  to  zero  as  we  moved  the 
eye  to  the  polar  axis.  An  eye  at  Q  would  see  the 
shining  point;  but  an  eye  at  P  would  see  nothing. 
This  is  contrary  to  experience  with  glowing  ma- 
terial points.  A  lighted  match  or  glowing  point, 
sends  out  rays  in  all  directions. 

The  discrepancy  is  accounted  for  by  the  fact 
that  the  polar  axis  of  the  electric  disturbance  in 
a  luminous  point,  supposed  to  be  due  to  oscillat- 
ing electrons,  is  constantly  shifting  its  direction 
in  space.  One  wave  may  have  its  polar  axis 
vertical,  but  after  a  few  more  have  passed  by, 
there  will  be  a  wave  with  its  axis  horizontal,  and 
later  again  the  axis  will  be  vertical;  so  that  in  a^ 
single  second  of  time  including  millions  of  mil-! 
lions  of  waves,  the  atomic  electric  oscillators  will 
have  turned  into  all  directions  and  will  have 
made  many  gyrations.  Consequently  the  eye 
will  have  received  in  that  time  many  waves  in 


72  WIRELESS  TELEGRAPHY 

their  equatorial  zone  and  also  many  in  their  polar 
zone,  so  that  the  average  effect  will  be  the  same 
in  every  direction.  We  need  only  suppose  the 
rod  oscillator  of  Figs.  10  and  20  rotated  about 
the  spark  center  in  all  directions  at  great  speed 
during  the  emission  of  a  long  train  of  waves,  to 
see,  in  imagination,  the  effect  that  would  be  pro- 
duced upon  the  eye  of  an  observer  at  any  distant 
point. 

Electromagnetic  waves  or  light  waves  in  which 
the  polar  axis  remains  fixed  in  space,  as  in  Fig. 
20,  are  called  plane- polarized  waves.  The  plane 
of  polarization  is  the  equatorial  plane  Q  O  Q? 
parallel  to  which  all  the  magnetic  flux-paths  are 
disposedc  Ordinary,  or  non-polarized,  light  may 
be  artificially  plane-polarized  by  optical  methods. 
We  may  say  then  that  ordinary  visible  light 
consists  of  electromagnetic  waves  of  sustained 
amplitude — t.e.9  not  merely  a  few  decaying  oscil- 
lations— within  a  certain  sharply  limited  range  of 
small  wave-lengths  or  high  frequencies,  and  with 
the  polar  axis  in  all  directions  in  rapid  succes- 
sion. Ordinary  daylight  contains  almost  all  the 
wave-lengths  within  the  visible  range,  showing 
that  vast  numbers  of  atomic  oscillators  of  differ- 
ent "  lengths "  are  simultaneously  operating  in 
the  glowing  solar  surface  and  are  mingling  or 
superposing  their  electromagnetic  waves.  These 


TJNGUIDED  ELECTROMAGNETIC  WAVES  73 

waves  reach  us  in  about  500  seconds  after  they 
leave  the  atomic  electric  oscillators  in  the  solar 
atmosphere. 

Solar  Wireless  Telegraph  Waves,  in  Broad  Sense, 

Necessary  to  Lije  oj  Human  Beings 
In  a  certain  sense,  therefore,  every  shining  star 
in  the  heavens  is  constantly  sending  out  spherical 
electromagnetic  waves  within  the  range  of  visual 
perception,  besides  probably  many  longer  waves, 
outside  of  that  range.  In  this  particular  sense 
we  are  constantly  receiving  wireless  telegraph 
waves  from  every  visible  orb,  and  the  message 
received  is  not  news  but  light.  Moreover,  since 
all  animal  energy  is  derived  from  plants,  and  all 
plants  build  up  their  substance  from  the  energy 
contained  in  the  sunlight  they  receive,  it  follows 
that  all  our  muscular  energy  is  derived  indirectly 
from  wireless  telegraph  waves  received  from  the 
sun. 

Union  of  Optics  with  Electromagnetics 

All  of  the  phenomena  of  light,  reflection,  re- 
fraction, polarization,  interference,  etc.,  which 
have  been  within  the  special  study  of  Optics 
for  many  decades,  have  in  recent  years  been 
imitated,  on  a  relatively  large  scale,  by  electro- 
magnetic waves  set  up  by  the  discharge  of  elec- 


74  WIRELESS  TELEGRAPHY 

trifled  conductors.  In  fact,  a  few  of  the  proper- 
ties of  optical  waves  which  are  difficult  to  detect, 
by  reason  of  the  excessively  short  optical  wave- 
length, are  more  easily  studied  and  revealed  in 
electromagnetic  waves  in  the  electrical  labora- 
tory. 

Classification  0}  Types  oj  Electromagnetic  Waves 

Summing  up  the  conclusions  reached  in  the 
last  few  chapters,  we  may  say  that  discharges 
between  two  rods  or  conductors  set  up  spherical 
waves.  Discharges  between  a  conductor  and  a 
plane  conducting  surface,  such  as  the  ground 
approximates,  set  up  hemispherical  waves. 
Waves  guided  between  a  pair  of  parallel  wires; 
and  between  an  aerial  wire  and  the  ground  are 
cylindrical  waves,  moving  end- wise.  The  waves 
employed  in  ordinary  wireless  telegraphy  are 
initially  hemispherical  waves  conforming  to,  or 
guided  by,  the  spheroidal  earth. 


CHAPTER   VIII 

PLANE   ELECTROMAGNETIC   WAVES 

Hemispherical  Waves  oj  Large  Radius  Are 
Virtually  Plane  at  Any  One  Point 

ANY  small  section  or  piece  cut  from  the  front 
of  a  hemispherical  wave  is  practically  flat,  or 
plane,  when  the  wave  is  remote  from  its  origin, 
just  as  the  earth's  spherical  surface  is  practically 
flat,  or  plane,  at  any  one  point  on  the  ocean,  be- 
cause its  radius  is  relatively  so  large.  Conse- 
quently, any  hemispherical  wave  advancing  over 
the  surface  of  the  earth  or  sea  may  be  regarded 
as  plane  locally.  It  comes  along  like  an  invisible 
upright  wall. 

A  section  of  a  single  such  wave  is  shown  in 
Fig.  21,  taken  along  the  line  of  march  V  V.  G  G 
represents  the  surface  of  the  ground.  The  elec- 
tric flux  rises  perpendicularly  at  P  P,  or  very 
nearly  so.  If  the  earth  conducted  perfectly,  the 
electric  flux  would  rise  strictly  vertical  from  it. 
Imperfect  conductivity  causes  a  wave  to  lean 
over,  or  bend  forward  slightly,  as  it  moves,  so 
that  a  perpendicular  to  the  wave  front  would  no 

75 


76  WIRELESS  TELEGRAPHY 

longer  lie  parallel  to  the  ground  but  would  point 
into  it.  For  practical  purposes,  however,  we 
may  take  the  electric  flux  as  perpendicular. 

In  the  front  half  of  the  wave,  we  have  taken 
the  flux  P  P  as  pointing  upwards,  corresponding 
to  a  moving  positive  charge  on  the  ground  be- 
neath ;  while  in  the  rear  half  these  conditions  are 
reversed.  This  relation  of  directions  depends 
upon  the  direction  of  the  fluxes  in  the  oscillator 
at  the  moment  that  this  particular  wave  was 
born. 

The  amplitude  of  the  current  waves  on  the 
ground  are  indicated  by  the  curved  line  f  p  o  n  r. 
The  line  of  zero  current  is  the  line  z  f  o  r  z.  The 
direction  of  the  magnetic  fluxes  is  also  indicated, 
by  circles  where  the  flux  is  directed  away  from 
the  observer  and  by  short  horizontal  bars  where 
towards  the  observer. 

The  wave  front  has  reached  F  F,  while  the 
rear  of  the  wave  is  at  R  R.  The  wave-length  is, 
therefore,  the  horizontal  distance  F  R.  At  the 
central  vertical  plane  O  O,  midway  between  the 
positive  and  negative  developments  of  the  wave, 
the  fluxes  are  zero  and  their  energies  are  con- 
sequently zero.  The  fluxes  are  densest  at  the 
central  planes  P  P  and  N  N,  and  their  energies 
in  a  given  volume  are  also  a  maximum  at  these 
planes. 


PLANE  ELECTROMAGNETIC  WAVES       77 

In  practice,  there  will  usually  be  a  train  of 
successive  waves  moving  over  the  ground  in 
place  of  the  solitary  wave  of  Fig.  21.  A  wave 


FIG.  21. — Section  of  a  Single  Electromagnetic  Wave 
Along  Line  of  Advance  and  Near  to  Surface  of  the 
Ground. 

train  in  wireless  telegraphy  does  not  usually  con- 
tain many  waves,  and  their  amplitude  succes- 
sively diminishes;  so  that  the  final  waves  in  the 
train  are  extremely  feeble. 

Analysis  oj  a  Single  Wireless  Telegraph  Wave 
At  and  Near  the  Earth 

A  section  of  the  wave  in  the  plane  of  P  P,  Fig. 
2i,  is  given  diagrammatically  in  Fig.  22.  It  ap- 
pears as  a  number  of  parallel  equidistant,  ver- 


78  WIRELESS  TELEGRAPHY 

tically  rising,  electric  flux  lines,  crossed  at  right 
angles  by  a  number  of  parallel  equidistant  hori- 
zontal magnetic  flux  lines.  This  means  that  both 
the  electric  and  the  magnetic  fluxes  have  uniform 
intensity  in  this  plane.  The  charge  moving  upon 


«r    «f-    *f    *f-     4-    «f-    *f    « 

4.  4.  4.  4.  4.  4.  4,  4.  4. 
4.  4.  4.  4V  4.  4.  4.  4V  4. 

4.4.4.4.4.4.4.4.4. 


4-4-4-4- 

-L 


FIG.  22. — Diagrammatic  Section  of  Plane  Vertical 
Electromagnetic  Wave  Parallel  to  Wave-Front 
and  Advancing  Towards  Observer,  with  Electric 
Flux  Rising  Vertically  from  Positive  Charge  on 
Ground  and  Magnetic  Flux  Horizontal. 

the  surface  of  the  ground  below  the  wave  is  posi- 
tive. Since  the  ground  is  not  a  perfect  conduc- 
tor, the  fluxes  penetrate  into  it  to  some  extent. 
This  causes  a  certain  amount  of  energy  to  be 
expended  in  the  penetrated  layer  of  soil  as  heat, 
derived  from  eddy  currents,  of  parasitic  electric 
currents,  in  the  soil.  The  energy  expended  at 


PLANE  ELECTROMAGNETIC  WAVES       79 

the  foot  of  the  wave  has  to  be  paid  for  from  the 
stock  of  energy  residing  in  the  wave  as  a  whole; 
so  that  energy  is  fed  downwards  as  the  wave  runs 
along,  causing  a  weakening  of  the  moving  fluxes, 
in  addition  to  the  weakening  caused  by  the 
simple  hemispherical  expansion  of  the  wave. 

Electric  and  Magnetic  Forces  Embodied  in  the 
Wave  and  Moving  Therewith 

If  we  could  compel  the  wave  to  stand  still  for 
a  moment,  instead  of  running  by  the  observer 
at  light-speed,  we  should  expect  to  find  that  a 
positively  electrified  pith  ball  would  be  urged 
upwards  by  the  upwardly  pointing  electric  flux 
of  the  wave  as  depicted  in  Fig.  22;  while  a  deli- 
cately poised  magnetic  compass  needle  would 
tend  to  align  itself  along  the  lines  M  m  in  the 
wave  front.  In  any  ordinary  wave,  however, 
these  electric  and  magnetic  forces  would  be  of 
very  feeble  magnitude.  The  fact  that  they  are 
able  to  produce  recognizable  effects  as  they  pass 
by  is  due  to  their  enormous  speed,  the  speed  of 
light  in  the  medium. 

Transparency  of  Electric  Non-conducting  Obsta- 
cles to  Long-wave  Light 

When  a  plane  electromagnetic  wave,  many 
meters  or  yards  long,  running  along  the  surface 


80  WIRELEvSS  TELEGRAPHY 

of  the  ground,  strikes  a  brick  wall,  or  a  •wooden- 
frame  house  devoid  of  metal,  it  passes  through 
these  obstacles  with  very  little  disturbance.  This 
means  that  if  our  eyes  were  capable  of  responding 
to  these  waves,  so  that  they  produced  the  sensation 
of  some  type  of  color,  such  non-conducting  obsta- 
cles would  be  transparent  to  that  color  of  light, 
and  we  could  look  through  a  brick  house  or  a 
wooden  house  without  difficulty,  when  objects 
were  illuminated  by  these  waves,  or  in  this 
imaginary  type  of  color.  If,  however,  the  ad- 
vancing waves  strike  an  electrically  conducting 
obstacle,  such,  for  example,  as  a  simple  vertical 
metallic  rod,  indicated  in  Fig.  23,  the  obstacle 
will  either  absorb  or  reflect  the  waves  and  will 
cast  a  shadow  beyond  it,  the  shadow  being,  of 
course,  invisible  to  us;  since  the  waves  are  invis- 
ible; but  the  shadow  can  be  determined  and 
mapped  out  by  suitable  electric  apparatus;  or 
by  what  may  be  called  an  artificial  eye. 

Shadow  Cast  by  a  Vertical  Electric  Conductor  in 
the  Path  of  an  Electromagnetic  Beam 

The  electric  flux  only  is  indicated  in  Fig.  23 
advancing  from  left  to  right,  over  the  ground 
G  G.  At  fl,  it  is  about  to  strike  the  vertical 
metallic  rod  A  B,  connected  with  the  ground. 
Such  a  vertical  might  be  a  leaden  water-pipe,  or 


PLANE  ELECTROMAGNETIC  WAVES       81 

a  copper  wire.  At  b  the  wave  has  passed  the 
vertical  and  a  gap  has  been  thereby  torn  in  the 
wave.  The  lower  edge  of  the  wave  at  the  rent  is 
bent  backwards  and  the  subsequent  direction  of 
movement  of  this  edge,  being  always  perpendicu- 
lar to  the  local  surface,  is  downwards  as  well  as 


1 
t 

I 

1  f  1  1  1 

J  1  1  4  * 

1  i  *  *  * 
I  1  *  '  * 

1 

r 
4, 
4 

4 

l 

| 

4 

1  1 

i 
i 

i 

1       4        1 

i! 

I 
I 

If 

4 
1 
| 

4 

4 

1 
t 

4 

i 

i 

i 
t 
t 

t  i 

1       t 

•4        t         • 

I 

ill  

i   i   i 

FIG.  23. — Diagram  Indicating  the  Electromagnetic 
Shadow  Cast  by  a  Vertical  Conductor  in  the  Path 
of  an  Advancing  Plane  Wave. 

to  the  right.  In  the  successive  positions  c  d  e  /, 
the  wave  is  spreading  down  from  above,  and  at 
m  the  rent  in  the  wave  net  has  been  repaired,  or 
the  shadow  behind  A  B  has  been  rilled  up,  the 
energy  of  the  flux  put  into  the  patch  being  drawn 
from  the  remainder  of  the  wave.  It  is  to  be 
understood,  however,  that  the  diagram  of  Fig.  23 
makes  no  pretensions  to  geometrical  accuracy, 
and  the  exact  contour  of  the  shadow  thrown  by 
a  conducting  obstacle  is  not  yet  determinable 
with  precision. 


82  WIRELESS  TELEGRAPHY 

After  the  wave  has  struck  the  rod  A  B,  a  dis- 
turbance is  reflected  back  from  the  rod  into  the 
region  A  B  n  G,  at  the  same  time  that  the  shadow 


*  > 


rr 


*  * 

*  * 


t  *  * 

lil 


t  f 

FIG.  24. — Diagrammatic  Sections  in  Elevation  and  Plan 
of  Electromagnetic  Wave  Striking  a  Vertical  Con- 
ductor while  Advancing  Towards  Observer. 

is  cast  beyond.     This  reflected  disturbance  is, 
however,  omitted  from  the  illustration. 

Fig.  24  presents  a  sketch,  both  in  elevation  and 
in  plan,  of  the  actions  occurring  when  the  wave 
strikes  the  vertical  conductor.  The  wave  is  sup- 
posed to  be  advancing  towards  the  observer.  It 
will  be  seen  that  the  electric  flux,  which  is  every- 


PLANE  ELECTROMAGNETIC  WAVES       83 

where  distributed  as  in  Fig.  22  (reversed),  before 
it  strikes  the  vertical  A  B,  is  drawn  in  on  each 
side  to  the  rod  and  converges  on  the  same,  con- 
tinuing to  run  down  the  rod  for  a  little  while 
after  the  wave  has  passed.  The  magnetic  flux 
is  shown  in  the  plan  at  the  base  of  the  illustration. 
Before  the  wave  reaches  the  rod,  as  at  m  m,  the 
magnetic  flux  lies  in  a  horizontal  straight  line, 
parallel  to  the  wave  front.  As  soon  as  the  wave 
strikes  the  rod,  the  magnetic  flux  bends  around 
it  clockwise,  and  also  descends  the  rod  at  light- 
speed.  S  S  is  the  shadow  cast  by  the  rod  B,  or 
the  space  denuded  of  magnetic  flux  for  a  certain 
distance  behind  B. 

Looking  at  the  action  from  another  standpoint, 
we  may,  in  the  light  of  our  provisional  electro- 
magnetic theory,  consider  that  the  electric  flux 
advancing  over  the  ground  brings  a  local  right- 
handed  torsional  stress  upon  the  ether,  which, 
by  electric  rigidity,  resists  the  stress  and  limits 
the  flow  of  ether  to  that  small  amount  found  in 
the  wave  front  as  indicated  by  the  horizontal 
magnetic  flux  lines  M  M,  Fig.  24.  At  soon  as 
the  electric  flux  strikes  the  conducting  rod  A  B, 
the  elastic  rigidity  of  the  ether  is  lost,  owing  to 
the  action  of  the  conductor  and  the  electrons 
residing  in  it.  The  ether  at  the  rod  gives  way 
before  the  stress,  and  flows  bodily  around  the  rod 


84  WIRELESS  TELEGRAPHY 

in  dense  magnetic  flux  streams.  On  such  a 
hypothesis,  a  conductor  behaves  like  a  gap  in  the 
ether,  and  the  advancing  electromagnetic  wave 
pours  electric  and  magnetic  fluxes  spirally  or 
vertically  down  into  the  gap  as  it  goes  by. 

Comparisons  Between  Reflection  of  Short  and 
Long  Waves  of  Light 

Whatever  hypothesis  we  adopt  to  assist  the 
mind's  eye  in  depicting  the  process,  we  must 
expect  to  find  the  action  similar  to  that  which 
occurs  when  half-micron  electromagnetic  waves, 
i.e.,  visible  light,  strike  an  opaque  obstacle. 
There  is  a  reflected  wave  train  thrown  back  by 
the  obstacle.  There  is  also  a  shadow  cast  be- 
hind it,  and  there  is  energy  absorbed  into  the 
substance  of  the  obstacle.  The  width  of  the 
shadow  cast  by  a  parallel  beam  of  light  is  appar- 
ently no  wider  than  the  obstacle;  whereas  in 
Fig.  24,  the  shadow  cast  is  indicated  as  being 
many  times  the  width  of  the  vertical  rod.  But 
it  has  to  be  remembered  that  if  the  optical 
shadow  of  a  rod  were  one  quarter  of  a  wave- 
length wider  than  the  rod,  the  difference  would 
be  only  about  a  sixth  of  a  micron  (Tg^o0ft) 
and  quite  insignificant;  whereas  if  the  rod  A  B 
had  the  same  height  as  the  simple  rod  oscillator 
which  originally  emitted  the  wave,  a  shadow  hav- 


PLANE  ELECTROMAGNETIC  WAVES       85 

ing  a  breadth  of  a  quarter  wave-length  would  be 
as  broad  as  the  height  A  B,  or  the  distance  N  N 
in  Fig.  24,  and  would  be  equal  to  the  height  A  B, 
a  very  appreciable  distance. 

Gashes  Torn  in  Electromagnetic  Waves  by  Verti- 
cal Conductors  on  the  Earth 

It  is  evident  that  upright  metallic  rods,  such 
as  lightning-conductors,  tear  rents  in  any  passing 
electromagnetic  wave  running  along  the  ground. 
On  the  other  hand,  a  conductor  parallel  to  the 
ground,  such  as  a  trolley-wire,  or  an  overhead 
telegraph  wire,  does  not  sensibly  affect  a  passing 
electromagnetic  wave.  Looked  at  in  another 
way,  a  vertical  rod  is  cut  by  the  rushing  hori- 
zontal magnetic  flux  at  light-speed,  and  acts  like 
a  single-wire  dynamo,  moving  through  a  very 
feeble  magnetic  field  at  the  speed  of  light.  Again, 
a  vertical  rod  picks  up  a  certain  difference  of 
electric  potential  between  the  electric  flux  at  its 
top  and  at  its  base.  In  either  of  these  ways,  the 
rod  becomes  the  seat  of  an  electric  impulse  or 
electromotive-] or ce  during  the  brief  interval  in 
which  the  wave  is  passing  by  it.  But  if  we  place 
the  rod  horizontal,  instead  of  vertical,  the  electric 
flux  in  the  wave  will  cut  the  rod  perpendicularly 
and  the  magnetic  flux,  in  cutting,  only  acts  upon 
the  thickness  of  the  rod;  so  that  the  electromo- 


86  WIRELESS  TELEGRAPHY 

tive  force  set  up  therein  by  the  passing  wave  will 
be  insignificantly  small,  and  will  be  directed 
transversely  or  across  the  diameter  of  the  hori- 
zontal rod. 

Accordingly,  when  a  single  electromagnetic 
wave  hits  a  vertical  rod,  a  rent  is  torn  in  the  wave, 
and  the  breadth  of  the  rent,  although  not  yet 
accurately  known,  may,  perhaps,  be  a  quarter 
of  a  wave-length.  The  energy  which  resided  in 
the  wave  within  the  region  torn  out,  is  available 
for  setting  up  electric  currents  in  the  rod,  after 
allowing  for  what  is  lost  by  reflection  and  second- 
ary radiation. 

It  may  be  readily  imagined  that  when  an 
electromagnetic  wave  strikes  a  steel  bridge,  or  a 
steel  sky-scraper  office-building,  it  casts  a  long 
shadow,  and  a  relatively  large  quantity  of  energy 
is  torn  out  of  the  wave.  Trees  also,  and  shrubs 
too,  in  lesser  degree,  have  been  found  to  be  feebly 
conducting,  and  it  is  believed  that  they  absorb 
energy  from  waves  passing  them.  This  fact 
taken  in  connection  with  the  imperfect  conduc- 
tivity of  dry  soil,  in  comparison  with  sea  water, 
accounts  for  the  considerably  greater  distance  at 
which  electromagnetic  waves  can  be  transmitted 
and  detected  over  the  ocean  than  over  land.  The 
signaling  distance  range  over  the  sea  is,  roughly, 
double  the  signaling  distance  range  across  country. 


PLANE  ELECTROMAGNETIC  WAVES       8; 

Elementary  Analysis  of  Electric  Oscillations  Set 
Up  in  a  Vertical  Conductor  by  the  Passage  of 
Waves 

It  is  important  to  notice  the  principal  events 
that  occur  in  the  neighborhood  of  the  vertical 
rod  after  it  has  been  struck  by  the  onrushing 
electric  wave.  Fig.  25  indicates  diagrammati- 
cally  nine  successive  stages  in  half  a  complete 
cycle  of  these  events.  The  line  of  crossed  ar- 
rows immediately  under  the  letters  ABC 
.  .  .  .  I,  represents  the  directions  of  electric 
and  magnetic  flux  in  the  advancing  wave  over 
the  rod.  Thus  at  A,  the  conditions  are  those 
indicated  in  Figs.  23  and  24;  namely,  the  electric 
flux  is  pointing  downwards  and  the  magnetic 
flux  pointing  to  the  right,  as  viewed  by  an  ob- 
server who  sees  the  wave  advancing  towards  him. 
At  E  and  F  these  fluxes  are  in  the  act  of  reversing 
through  zero,  corresponding  to  a  plane  such  as 
O  O  in  Fig.  21.  At  I,  the  fluxes  have  completely 
reversed. 

Underneath  each  diagram  of  a  rod  section  in 
Fig.  25,  there  appears  a  plan  view  showing  the 
direction  of  magnetic  flux  in  the  wave  just  before 
striking  the  rod,  and  also  of  magnetic  flux  en- 
circling the  rod.  Thus,  at  A,  the  magnetic  flux  in 
the  air  is  at  full  development  towards  the  right 


88  WIRELESS  TELEGRAPHY 

hand  of  the  observer,  while  around  the  rod  it  is 
clockwise.  At  C,  the  clockwise  magnetic  flux 
encircling  the  rod  has  reached  full  development, 
or  the  electric  current  over  it  is  a  maximum. 
Between  E  and  F  the  magnetic  flux  in  the  air 
reverses  or  passes  through  zero.  At  G,  the 
magnetic  flux  encircling  the  rod  passes  through 
zero.  At  I,  the  magnetic  flux  in  the  air- wave  has 
developed  completely  in  its  reverse,  or  left- 
handed  direction. 

Examining  the  rod  at  A,  it  will  be  seen  that 
the  electric  flux  of  the  passing  wave  has  con- 
verged upon  it,  as  already  seen  in  Figs.  23  and 
24.  This  flux  immediately  starts  to  run  down 
the  rod  to  ground,  as  indicated  by  the  long  dotted 
arrow.  The  instant  it  begins  to  run,  the  electric 
flux  reverses  direction,  or  assumes  the  outward 
direction  shown  at  B,ihe  magnetic  flux  remaining 
clockwise,  as  viewed  from  above.  As  soon  as 
the  flux  reaches  the  conducting  ground  at  the 
base  of  the  rod,  it  is  reflected  thence  upward,  with 
a  new  reversal  of  electric,  and  maintenance  of 
magnetic,  flux  direction.  At  E,  the  stream  of 
flux  on  the  rod  is  about  to  reach  the  top.  At  the 
top,  the  flux  reverses  magnetically,  or  is  reflected 
downward,  with  persistence  of  inward  electric 
flux.  At  G,  the  magnetic  flux  is  half  clockwise 
and  half  counter-clockwise,  representing  zero  of 


89 


QO  WIRELESS  TELEGRAPHY 

current,  but  maximum  electric  potential.  At  1, 
the  flux  is  in  full  descent  again,  with  counter- 
clockwise magnetic  field. 


Resonance  in  Electric    Conductors  Struck  by 
Wave-trains 

We  have  purposely  chosen  the  length  of  the 
rod  as  one  quarter  of  the  length  of  the  plane  wave 
advancing  through  the  air.  This  brings  about 
such  an  adjustment  of  the  motion  of  flux  over 
the  rod  as  enables  the  next  succeeding  wave  to 
add  to,  or  increase,  the  movement.  If  we  ex- 
tended the  diagram  of  Fig.  25  through  eight  more 
such  phases  we  should  return  to  the  original  con- 
dition at  A,  when  the  flux  in  the  next  wave  would 
not  only  repeat  the  cycle,  but  would  also  increase 
the  amplitude.  If  the  rod  conducted  perfectly, 
and  also  the  ground  at  its  base,  each  wave  as  it 
arrived  through  the  air  would  add  to  the  fluxes 
running  up  and  down  the  rod,  on  the  familiar 
principle  of  the  child's  swing,  whose  oscillations 
may  be  increased  by  timing  the  pushes  to  the 
natural  period  of  oscillation.  In  this  case,  how- 
ever, the  swing  of  the  rod  is  adjusted  by  its  length, 
so  as  to  be  in  rhythm  to  the  train  of  arriving 
waves.  Such  a  condition  of  coincidence  between 
the  times  of  arrival  of  the  successive  wave- crests, 


PLANE  ELECTROMAGNETIC  WAVES       91 

and  the  natural  time  of  electric  oscillation  of  the 
rod,  is  called  electric  resonance. 

If  we  could  obtain  a  very  long  train  of  uniform 
advancing  waves  and  adjust  the  length  of  the 
vertical  rod  into  resonance  therewith,  retaining 
perfect  conduction,  the  fluxes  running  up  and 
down  would  increase  indefinitely,  were  it  not  for 
secondary  radiation.  That  is,  the  rod,  excited  in 
this  way  by  arriving  waves,  would  become  an 
oscillator  in  its  turn,  and  discharge  the  energy  it 
received  in  a  new  series  of  radiating  waves,  as  in 
Fig.  15.  In  practice,  however,  the  waves  re- 
ceived through  the  air  have  such  feeble  amplitude, 
they  decay  so  soon,  the  number  in  a  train  is  so 
small,  and  the  conductivity  of  the  rod  and  ground 
base  is  so  far  from  being  perfect,  that  even  when 
the  rod  length  is  adjusted  into  resonance,  the 
currents  developed  over  the  rod,  as  in  Fig.  25,  are 
comparatively  feeble.  The  secondary  radiation 
is,  therefore,  insignificant. 

If  the  length  of  the  rod  is  in  the  opposite  con- 
dition to  that  required  for  resonance,  the  fluxes 
generated  thereon  by  the  first  wave  will  be  op- 
posed, instead  of  aided,  by  the  fluxes  generated 
in  the  second,  and  so  on.  Consequently,  there 
will  be  comparatively  feeble  currents  set  up  on 
the  rod.  If,  however,  the  length  of  the  rod  is 
adjusted  for  resonance,  there  will  be  a  building 


92  WIRELESS  TELEGRAPHY 

up  of  electric  current  on  the  rod,  unless  the  arriv- 
ing wave  train  is  too  short,  or  unless  the  electric 
obstruction  and  want  of  conductivity  in  rod  and 
ground  suppress  the  development. 

In  order  to  adjust  the  rod  to  the  resonant  con- 
dition, it  is  not  always  necessary  to  alter  its  height. 
The  virtual  length  can  be  altered  by  the  insertion 
of  a  suitable  form  of  conductor  or  wire  at  the 
base,  between  rod  and  ground,  in  a  manner  to  be 
described  later.  In  such  a  manner  the  time  of 
oscillation  of  a  rod  can  be  altered  without  chang- 
ing the  actual  height. 

Resume  o]  Conditions  Attending  the  Impact  of 
Waves  Against  Vertical  Conductors 

Summing  up  the  above  results,  we  find  that  a 
vertical  conductor  connected  with  good  conduct- 
ing ground,  and  set  up  anywhere  in  the  path  of  a 
train  of  electromagnetic  waves,  will  have  alternat- 
ing, or  to-and-fro  electric  currents  set  up  on  it, 
the  energy  contained  in  these  currents  being  the 
energy  in  the  up-and-down  moving  fluxes,  which 
energy  is  drawn  from,  or  scooped  out  of,  the 
arriving  electromagnetic  waves  as  they  pass  by. 
These  alternating  currents  in  the  rod  are  capable 
of  being  built  up,  or  successively  increased  in 
strength,  by  the  impulses  of  the  successive  waves, 
if  there  be  resonance,  i.e.,  if  the  natural  time  of 


PLANE  ELECTROMAGNETIC  WAVES       93 

oscillation  of  the  rod  be  the  same  as  the  periodic 
time  of  the  arriving  waves.  For  a  simple  vertical 
rod,  devoid  of  inserted  apparatus,  this  will  be 
when  its'  height  is  one  quarter  of  the  wave-length, 
and  therefore  equal  to  the  height  of  the  simple 
vertical  rod  oscillator  which  is  capable  of  originat- 
ing such  waves.  In  other  words,  if  the  arriving 
waves  have  been  produced  by  a  distant  simple 
rod  oscillator,  resonance  will  require  the  heights 
of  the  oscillator  and  of  the  receiver  verticals  to 
be  equal.  Resonance  would  be  capable,  theo- 
retically, of  setting  up  an  indefinitely  great  ampli- 
tude in  a  perfectly  conducting  receiver  rod,  set  in 
perfectly  conducting  ground,  with  a  constantly 
maintained  succession  of  waves  at  the  oscillator, 
were  it  not  for  secondary  radiation  of  waves  from 
the  receiver.  In  practice,  however,  the  alternat- 
ing currents  set  up  at  the  receiver  may  be  ma- 
terially increased  by  bringing  the  receiving  rod 
into  resonance,  but  the  development  is  arrested 
by  imperfect  conduction  at  the  receiver,  as  well 
as  by  discontinuity  of  the  oscillations,  or  small 
trains  of  waves  at  the  oscillator.  Moreover,  the 
insertion  of  a  receiving  instrument  into  the  path 
of  the  vertical  receiver  rod  also  causes  energy  to 
be  absorbed,  and  interferes  with  the  production 
of  resonant  increase  of  oscillations. 


94  WIRELESS  TELEGRAPHY 

Energy  oj  Electric   Oscillations,   or   Oscillating 
Currents,  Set  Up  in  a  Vertical  Receiver 

The  energy  which  is  available  for  producing 
such  electric  currents  by  any  wave  at  the  receiver 
depends  upon  the  energy  in  the  entire  hemispheri- 
cal wave  at  that  moment.  It  will  evidently  be 
but  a  very  small  fraction  of  the  total  energy  of  the 
wave,  when  the  receiver  is  far  from  the  oscillator, 
since  the  area  of  the  wave  which  can  come  into 
contact  with  the  receiving  rod,  or  into  its  region 
of  influence,  is  so  small.  If  we  suppose  that  the 
receiver  has  a  height  of  a  quarter  wave-length, 
for  resonance,  and  that  the  effective  breadth  of 
area  from  which  energy  is  drawn,  as  in  Fig.  23, 
is  also  a  quarter  wave-length,  then  the  fractional 
part  of  the  wave's  energy  available  for  producing 
electric  current  at  the  receiver,  is  the  square  of 
the  height  of  the  receiver  rod,  divided  by  the 
superficial  area  of  the  hemisphere  occupied  by 
the  entire  wave  at  the  instant  it  strikes  the  rod. 
For  example,  if  we  suppose  that  a  certain  wave 
in  a  series  emitted  by  an  oscillator  contains  at  the 
moment  of  shipment  an  amount  of  energy  equal 
to  i  kilogramme-meter  (7.24  foot-pounds,  or  the 
work  done  in  lifting  one  pound  through  a  vertical 
height  of  7.24  feet),  then  a  quarter- wave  vertical 
receiving  rod  at  a  distance  of  30  kilometers 


PLANE  ELECTROMAGNETIC  WAVES       95 

(18.6  miles),  with  a  height  of  say  31.6  meters 
(103.6  feet)  might  perhaps  absorb  energy  from 
the  wave  as  it  passed,  over  a  height  of  31.6  meters 
and  a  breadth  of  31.6  meters,  or  an  area  of  wave 
surface  amounting  to  1,000  square  meters 
(10,760  square  feet).  But,  neglecting  the  curva- 
ture of  the  earth,  the  area  of  a  hemisphere  30 
kilometers  (18.6  miles)  in  radius  would  be  5,655 
millions  of  square  meters  (60,800  millions  of 
square  feet);  so  that  the  electromagnetic  energy 
capable  of  being  drawn  on  to  the  rod  would  be 
CTTiTTO*  a  kilogramme-  meter, 


or  1  7  ergs.  The  received  energy  should  be  about 
60  per  cent,  greater  than  this,  because  of  the 
greater  density  of  flux  and  energy  in  the  equa- 
torial zone  of  the  transmitted  wave,  i.e.,  near 
the  earth's  surface.  On  the  other  hand,  how- 
ever, a  distinct  reduction  would  have  to  be  made 
for  the  energy  wasted  in  transmission  along  the 
surface  of  the  soil,  by  reason  of  the  earth's  im- 
perfect conductivity,  or  for  other  vertical  con- 
ductors, such  as  metallic  structures,  or  trees, 
intervening  between  oscillator  and  receiver. 

Our  knowledge  is  still  very  imperfect  as  to  the 
effective  surface  area  drawn  upon  by  a  vertical 
rod,  and  also  as  to  the  amount  of  energy  drained 
from  the  feet  of  an  advancing  hemispherical  wave 
by  reason  of  the  earth's  imperfect  conductivity. 


96  WIRELESS  TELEGRAPHY 

This  loss  is  known  to  be  greater  after  dry  weather 
than  after  rain.  It  is,  however,  clear  that  accord- 
ing to  the  working  theory  outlined  above,  the 
energy  capable  of  being  received  by  any  such  ver- 
vertical  rod  increases  as  the  square  of  its  height, 
assuming  that  the  resonant  condition  is  main- 
tained, and  also  inversely  as  the  square  of  the 
distance  between  the  sending  and  receiving  sta- 
tions. The  total  energy  available  for  producing 
alternating  electric  currents  at  the  receiver  will 
be  the  sum  of  the  successive  fractional  amounts 
drawn  from  each  single  wave  in  turn,  assuming 
that  the  successive  effects  can  be  prevented  from 
canceling  or  annulling  each  other,  by  the  adjust- 
ment of  the  receiver  to  the  resonant  condition. 


CHAPTER   IX 

THE    SIMPLE    ANTENNA    OR    VERTICAL    ROD 
OSCILLATOR 

The  Antenna  and  Transmitting  Apparatus 

WE  have  already  arrived  at  the  conclusion 
from  preceding  chapters  that  wireless  telegraphy 
ordinarily  employs  hemispherical  electromagnetic 
waves  emitted  from  a  vertical  rod  oscillator  or 
antenna,  in  short  trains  of  from  two  to  thirty 
waves  of  successively  diminishing  amplitude. 
On  or  near  the  ground,  or  sea  level,  and  at  a 
great  distance  from  the  transmitting  station, 
these  waves  are  for  all  practical  purposes  plane 
waves,  advancing  over  the  conducting  ground, 
or  sea,  with  the  speed  of  light,  in  a  direction 
radial  to  the  sending  station,  after  allowing  for 
the  curvature  of  the  earth.  We  now  proceed  to 
consider  the  essential  elements  of  the  antenna, 
and  of  the  apparatus  employed,  at  the  trans- 
mitting station. 

The  simplest  type  of  vertical  antenna  or  rod 
oscillator  is  represented  in  Fig.  26.  It  consists 
essentially  of  a  single  vertical  metallic  wire  A  B, 
97 


WIRELESS  TELEGRAPHY 


suspended  from  an  insulator  I,  which  is  sup- 
ported from  a  wooden  mast  structure  indicated 
in  dotted  lines.  This  vertical  wire,  air- wire, 
aerial,  or  antenna,  is  insulated  from  the  ground 


FlG.  26. — Essential  Elements  of  a  Simple  Vertical  Os- 
cillator, or  Antenna,  for  Emitting  Hemispherical 
Electromagnetic  Waves. 

by  the  air-gap  G,  so  long  as  it  is  electrically  in- 
active. The  lower  terminal  of  the  air-gap  com- 
municates with  a  metallic  plate  P  sunk  in  moist 
earth,  or  below  low-tide  level,  if  on  the  seashore. 
Sometimes  bare  wires  w  w  are  laid  out  radially 
from  the  ground  wire  in  various  directions,  at  or 


THE  SIMPLE  ANTENNA  99 

near  the  surface  of  the  ground,  so  as  to  improve 
the  local  conductivity  of  the  soil,  and  help  to 
form  a  good  electric  mirror  at  the  ground  surface 
s  s  s,  from  which  the  waves  may  be  reflected  back 
and  up  the  antenna,  not  only  at  the  conductor,  but 
in  its  vicinity,  as  indicated  in  Fig.  15. 

The  length  of  the  wave  emitted  by  a  simple 
vertical  wire  antenna  as  shown  in  Fig.  26  is  be- 
lieved to  be  very  closely  four  times  the  height  of 
the  antenna  A  B  G  S.  Thus,  if  the  antenna  had 
a  height  of  30  meters  (32.8  yards),  above  perfect 
ground,  the  length  of  the  waves  sent  out  would  be 
120  meters  (131.2  yards).  The  number  of  such 
waves  which  would  cover  the  distance  travelled 
by  light  in  one  second  would  be  AHJfcfJJJUUL= 
2,500,000;  so  that  there  would  be  two  and  a  half 
millions  of  such  waves  occupying  one  second,  if 
the  oscillator  could  be  kept  at  work  for  that  time. 
This  means  that  the  frequency  of  the  waves 
would  be  2,500,000  cycles  per  second,  or  the 
time  occupied  by  any  one  complete  wave  to  pass 
a  given  point  would  be  3,5  o  o;o  o  otn  second.  If 
we  call  the  one  millionth  part  of  a  second,  one 
microsecond  for  convenience  of  description,  then 
one  complete  wave  would  pass  off  in  -$=%  micro- 
second. Since  each  wave  contains  both  a  posi- 
tive and  a  negative  impulse,  either  impulse  would 
pass  by  in  \  of  a  microsecond. 


1C*,  WIRELESS  TELEGRAPHY 

The  Large  Activity  of  an  Antenna 

Owing  to  this  extremely  short  period  of  oscil- 
lation, antennas  are  remarkable  for  their  activity 
or  power.  The  amount  of  energy  which  can  be 
stowed  away  in  a  simple  vertical  antenna  as 
electric- flux  energy  in  the  surrounding  ether,  by 
charging  it  to  a  suitably  high  voltage,  is  compara- 
tively small,  being  usually  not  more  than  20 
gramme-meters  (0.14  foot-pound),  or  the  work 
done  in  lifting  20  grammes  to  a  height  of  one 
meter.  When  this  energy  is  released,  by  the 
discharge  of  the  antenna  across  the  spark-gap 
G,  Fig.  26,  part  of  this  energy  is  expended  in  the 
heat  of  the  spark  and  in  heating  the  surface  of 
the  conducting  antenna.  The  remainder  is 
available  for  radiation  as  a  series  of  electro- 
magnetic waves.  Perhaps  not  more  than  3 
gramme-meters  (0.022  foot-pound)  of  energy  will 
be  shipped  off  in  any  single  wave.  Nevertheless, 
this  energy  is  shot  off  by  this  particular  antenna 
in  -^5-  part  of  a  microsecond,  and  the  average 
rate  of  power  radiation  during  this  brief  interval 
will  thus  be  7,500,000  gram-meters  per  second, 
or  7,500  kilogramme-meters  per  second,  or  about 
100  horse-power. 

With  the  aid  of  auxiliary  apparatus,  an  antenna 
may  be  capable  of  radiating  electromagnetic  wave 


THE  SIMPLE      NTENA'   J        JjJ  rot 


energy  at  the  rate  of  hundreds  -  of 
but  only  for  a  few  microseconds  at  a  time,  so 
that  its  average  power  in  one  second,  or  in  one 
minute,  during  its  operation,  may  be  only  a 
small  fraction  of  a  horse-power.  An  antenna  of 
the  simple  type  shown  in  Fig.  26,  looks  like  a 
very  simple  and  innocent  machine;  but,  when 
thrown  into  electric  vibrations,  it  may  throw  out 
as  much  power  as  it  takes  to  operate  a  high- 
speed electric  locomotive;  only  it  does  not  keep 
the  power  up.  The  case  is  somewhat  similar 
to  that  of  a  revolver,  which  is  being  fired,  say, 
three  times  per  second.  At  each  explosion  the 
power  of  the  machine  is  relatively  very  great; 
but  between  shots  the  power  falls  to  nil;  so  that 
the  average  for  one  second,  the  power  of  the 
machine,  or  its  mean  rate  of  throwing  energy  off, 
is  comparatively  low. 

In  practice,  single-wire  antennas  are  seldom 
used,  and  multiple-  wire  antennas  are  customary. 
The  purpose  of  employing  a  plurality  of  conduc- 
tors is  two-fold.  In  the  first  place,  the  larger 
surface  of  the  antenna  gives  more  electric  flux 
in  the  air  when  charged,  and  this  increases  the 
stock  of  energy  held  by  the  antenna  prior  to  re- 
lease and  radiation.  In  the  second  place,  the 
larger  surface  permits  of  a  more  free  active  radia- 
tion or  discharge  of  electromagnetic  waves  into 


WIRELESS  TELEGRAPHY 


ir,  Independently  of  the  amount 
of  energy  to  be  released. 

Cylindrical  Antennas 

Fig.  27  represents  the  cylindrical  type  of  verti- 
cal antenna,  one  having  four  parallel  vertical 
t  wires  and  the  other  sev- 

enteen. Any  convenient 
number  may  be  used. 
The  wires  are  usually 
soldered  to  two  or  more 
metallic  hoops  H  H, 
which  not  only  strength- 
en the  structure  mechan- 
ically, but  also  keep  the 
oscillations  symmetrical 
electrically.  The  diam- 
eter of  these  hoops  may 
range  from  30  centime- 
ters to  several  meters  (i 
foot  to  several  yards).  If 
the  component  vertical 
wires  are  not  further 
than,  say,  half  a  meter 

apart  (19.7  inches)  these  bird-cage  cylinders  are 
almost  equivalent,  electrically,  to  complete  sheet- 
cylinders  of  metal.  The  bird-cage  cylinder  of 
multiple  parallel  wires  is  of  course  far  superior 


FIG.  27.— Types  of  Cylin- 
drical Frame  Vertical 
Oscillators. 


THE  SIMPLE  ANTENNA  103 

mechanically  to  a  complete  sheet-cylinder  or 
large  pipe,  both  in  cost,  lightness  and  freedom 
from  wind-pressures.  These  cylindrical  metallic 
frames  may  be  supported  by  suitable  insulators 
from  a  mast-arm  at  I,  their  lower  extremities 
G  leading  to  spark  gaps. 

In  some  instances  a  cylindrical  antenna  is 
formed  of  a  rigid  vertical  steel  tube,  bolted  to- 
gether in  sections  and  supported  on  insulators  at 
the  base.  The  tube  or  cylinder  is  prevented 
from  falling  by  guy- ropes  running  in  various 
directions,  and  in  which  insulators  called  strain- 
insulators,  because  they  are  subjected  to  tension, 
are  inserted  at  some  suitable  point  or  points. 

Harp,  Fan  and  Inverted  Cone  Antennas 

Other  forms  of  antenna  in  use  are  outlined  in 
Figs.  28  and  29.  The  former  indicates  the  harp 
type.  This  is  conveniently  supported  between 
two  wooden  masts,  as  indicated  in  dotted  lines, 
but  a  single  mast  may  serve  if  the  wires  are  sus- 
pended from  a  horizontal  arm.  The  five  wires 
shown  are  connected  at  the  top  and  at  the  bot- 
tom by  horizontal  wires.  The  entire  conducting ' 
frame  is  supported  by  insulators  at  1 1 1 1.  The 
harp  is  connected  to  the  spark-gap  by  the  wire  G, 

The  fan-shaped  antenna  of  Fig.  29  is  some- 
times used  on  board  ship.  The  stout  wire  I  I  is 


104 


WIRELESS  TELEGRAPHY 


strung  between  the  masts  B  A  and  D  C,  being 
supported  by  end-insulators  I  I.     The  descend- 


ib 


FIG.  28. 


FIG.  29. 


FIGS.  28  and  29. — Types  of  Harp-Shaped  and  Fan- 
Shaped  Antennas. 

ing  wires  are  each  connected  to  the  top-wire  I  I 
above  and  to  the  central  point  below;  whence  a 
wire  G  runs  to  the  spark- 
gap  and  ground,  or  on  a 
steamer  to  the  metallic 
frame  of  the  hull. 

At  some  stations  a  ser- 
ies of  fans  are  connected 
together  into  an  inverted 
cone,  as  seen  in  Fig.  30. 

FIG.  30.  — Typeof  Inverted  Here  four  masts  support 
Cone  Antenna.  . 

a    metallic    rectangle, 

through  insulators  not  shown  in  the  diagram. 
Metallic  wires  drop  from  the  rectangle  at  inter- 


THE  SIMPLE  ANTENNA  105 

vals  to  the  central  point  O,  whence  a  wire  runs 
across  a  spark-gap  to  ground. 

Whatever  the  form  of  the  antenna,  cylindrical, 
harp-shaped,  fan-shaped  or  conical,  the  object 
sought,  already  mentioned,  is  to  increase  the 
electric  flux,  and  electric  energy  associated  there- 
with, in  the  charge  of  the  antenna,  and  also  to 
facilitate  the  emission  of  the  waves  into  space  at 
the  recoils  from  the  upper  end  or  ends  of  the 
antenna. 

Electric  Oscillations  on  Antennas  Skin  Deep 

The  thickness  of  the  individual  wires  forming 
the  antenna  is  of  secondary  importance»  It  is 
the  surface  of  the  wires  which  is  of  principal  con- 
sideration. The  high-frequency  electric  cur- 
rents, or  oscillations,  running  up  and  down  the 
antenna,  are  not  able  to  penetrate  below  a  cer- 
tain skin  depth  into  the  conductor,  say  i  mm. 
The  higher  the  frequency,  the  less  the  penetra- 
tion, and  the  thinner  the  effective  conducting 
skin.  The  wires  are  usually  of  copper,  and 
about  4  mm  (J  inch)  in  diameter. 

Other  things  being  equal,  the  higher  the 
antenna,  of  whatever  form,  the  more  electric 
flux,  charge,  and  energy  it  will  hold;  so  that  the 
power  it  can  release  is  greater.  At  the  same 
time  the  length  of  the  wave  tends  to  be  greater. 


106  WIRELESS  TELEGRAPHY 

Sources  o)  Energy  jor  Feeding  to  an  Antenna 

The  source  of  electric  energy  for  charging  the 
antenna  is  generally  an  induction  coil,  or  spark 
coil,  excited  either  by  a  dynamo,  or  by  a  voltaic 
battery.  If  a  voltaic  battery  is  used,  it  is  com- 
monly a  secondary  ^  or  storage  battery?  charged 
by,  and  receiving  energy  from,  a  dynamo.  Con- 
sequently, while  it  might  be  possible  to  use  any 
electric  source  of  energy,  such  for  example  as  a 
frictional  machine;  yet,  in  practice,  the  energy  is 
furnished  by  a  dynamo  driven  by  water-power, 
steam-power,  or  gas.  An  ideal  form  of  dynamo 
exciter  would  be  an  alternating- cur  rent  dynamo 
which  generated  to-and-fro  electric  currents,  or 
currents  of  successively  reversing  directions,  with 
a  frequency  precisely  that  required  for  setting  up 
resonance  in  the  antenna.  If  such  a  very  high- 
frequency  dynamo  could  be  constructed  conveni- 
ently, it  would  be  capable  of  keeping  the  antenna 
in  full  oscillation  indefinitely.  That  is,  if  the 
radiating  power  of  the  antenna  were  say  300  kilo- 
watts or  400  horse-power,  it  would  be  possible  to 
connect  a  dynamo  of  at  least  300  kilowatts  capa- 
city (400  H  P)  to  the  antenna,  and  keep  it  con- 
stantly in  action  at  that  rate.  Such  a  dynamo 
would  have,  however,  to  generate  alternating 
currents  with  a  frequency  either  of  millions,  or, 


THE  SIMPLE  ANTENNA  107 

at  least,  many  thousands  of  cycles  per  second; 
whereas  the  dynamos  used  in  electric  lighting 
and  power  transmission  ordinarily  only  generate 
alternating  currents  with  a  frequency  of  sixty 


FIG.  31. — Induction  Coil  for  Generating  a  High  Voltage. 

(60)  cycles  per  second.  This  frequency  is  at 
least  hundreds  of  times  too  low  for  direct  exci- 
tation. 

Under  present  conditions  it  is  customary  to 
charge  the  antenna  by  an  induction  coil  of  some 
kindo  When  the  energy  is  supplied  by  a  storage 
battery,  an  induction  coil  is  used  resembling  that 
shown  in  Fig.  31.  This  apparatus,  which  is 
essentially  a  powerful  spark-coil,  has  a  central 


108  WIRELESS  TELEGRAPHY 

core  ot  iron,  in  the  form  of  a  bundle  of  iron  wires. 
There  are  two  coils,  or  windings,  of  insulated 
wire  placed  on  the  iron  core.  These  two  wind- 
ings are  carefully  insulated  from  the  core,  and 
from  each  other.  One  is  the  primary  winding, 
consisting  of  comparatively  few  turns  of  coarse 
cotton-covered  copper  wire.  The  other  is  the 
secondary  winding  of  very  many  turns  of  fine 
silk- covered  wire.  The  primary  wires  are  led 
out  at  p  p  and  the  ends  of  the  long  fine  secondary 
winding  are  connected  to  the  discharge  knobs 
s  s.  When  a  strong  current  is  flowing  steadily 
through  the  primary  winding,  supplied  by  an 
external  storage  battery,  there  will  be  no  electric 
impulse,  or  electromotive  force,  in  the  secondary. 
There  will,  however,  be  a  powerful  stationary 
magnetic  flux  distribution  surrounding  the  pri- 
mary current,  and  linked  with  the  secondary  coil. 
If  the  primary  current  be  now  suddenly  inter- 
rupted, the  magnetic  flux  linked  with  the  coils 
will  collapse  and  disappear.  In  so  doing,  how- 
ever, its  movement  generates  a  brief  but  very 
powerful  electric  impulse  in  the  secondary  wind- 
ing, constituting  a  powerful  electromotive  force, 
or  a  high  voltage,  i.e.,  a  voltage  capable  of 
jumping  across  a  considerable  distance  of  air- 
space. Other  things  being  equal,  the  length  of 
the  air-gap  across  which  a  spark  will  jump  is  an 


THE  SIMPLE  ANTENNA  109 

indication  of  the  magnitude  of  the  electromotive 
force  or  voltage  producing  the  spark. 

Similarity  of  Process  of  Transferring  Energy  in 
Induction  Coil  to  Wireless  Transmission 

The  conditions  which  accompany  the  trans- 
mission of  electric  power  from  the  primary  to 
the  secondary  winding,  a  distance  of  a  few  milli- 
meters or  centimeters  (a  few  tenths  of  an  inch 
up  to  an  inch  or  two),  resemble  those  which  ac- 
company the  transference  of  electric  energy  from 
the  sending  to  the  receiving  antenna.  Whereas, 
however,  in  the  latter  case,  the  distance  between 
the  primary  and  secondary  wires  is  relatively 
very  great,  and  the  energy  is  transferred  from 
one  place  to  the  other  stowed  away  in  a  wave  or 
series  of  waves;  in  the  former  case  of  the  induc- 
tion coil,  the  wave  has  no  room  to  develop  a 
separate  existence,  but  the  electromagnetic  fluxes 
are  linked  with  both  circuits  throughout  the 
process.  For  the  same  reason,  the  efficiency  of 
the  transmission  is  enormously  greater  in  the  in- 
duction coil  than  in  the  wireless  case.  Nearly 
all  of  the  electric  energy  leaving  the  primary 
winding  is  absorbed  by  the  secondary  winding. 
On  the  contrary,  nearly  all  of  the  electric  energy 
leaving  the  primary  antenna  goes  off  into  space, 
or  else  is  ultimately  absorbed  in  the  ground,  and 


110  WIRELESS  TELEGRAPHY 

hardly  any  is  absorbed  by  the  secondary  antenna. 
In  the  rough  calculation  given  on  page  95,  Chapter 
VIII,  it  appeared,  for  example,  that  only  i  part 
in  5,655,000  of  the  energy  liberated  by  the  oscil- 
lator, or  sending  antenna,  was  picked  up  by  the 
receiving  antenna,  under  the  conditions  there 
considered. 

Elements  o]  Sending  Apparatus  for  Producing 
Electromagnetic  Waves 

The  elements  of  the  connections  at  a  wireless- 
telegraph  sending  station  are  illustrated  in  Fig. 
32.  A  A  is  the  antenna,  or  the  wire  connecting 
therewith.  C  is  the  induction  coil.  The  pri- 
mary circuit  is  marked  in  full  lines  and  the  second- 
ary in  broken  lines.  The  primary  circuit  com- 
prises the  primary  winding  of  the  coil  C,  the 
voltaic  battery  B,  a  hand  key  K,  and  an  electro- 
magnetic vibrator  or  interrupter  V.  The  vibra- 
tor may  be  a  separate  piece  of  apparatus  included 
in  the  primary  circuit ;  or  it  may  form  part  of  the 
induction-coil  mechanism  as  shown.  It  is  essen- 
tially a  vibrating  circuit-maker-and-breaker  like 
the  vibrator  of  the  ordinary  electric  bell.  Its 
purpose  is  to  interrupt  the  primary  circuit  auto- 
matically and  rhythmically,  as  long  as  the  key  K 
is  depressed.  The  vibrator  V  may  give  inter- 
ruptions at  the  rate  of  say  200  cycles  per  second. 


THE  SIMPLE  ANTENNA 


ill 


It  dlso  gives  a  musical  note  or  tone  in  the  sur- 
rounding air,  corresponding  to  its  frequency  of 
vibration.  At  each  interruption  of  the  primary 
circuit  at  the  vibrator  V,  there  is  a  sudden  elec- 
tric impulse  generated  in  the  secondary  circuit, 


! 

! 

_ 

f  J       j 

^       !A 

» 

•••< 

o 

B 

)'        . 

takj 

»  —  i 

j 

FIG.  32. — Elements  of  Connections  at  Sending  Station. 

and  this  travels  up  the  antenna  at  light- speed. 
If  the  spark  gap  g  did  not  break  down,  there 
would  be  a  reflection  of  the  impulse  from  the  top 
of  the  antenna,  accompanied  by  an  electro- 
magnetic impulse  or  radiation  into  space;  but 
there  would  be  no  succession  of  waves  and  no 


112  WIRELESS  TELEGRAPHY 

proper  development  of  electromagnetic  wave 
emission.  If,  however,  the  impulse  on  its  return 
from  the  top  of  the  antenna  is  able  to  break  down 
the  air-gap  g  in  a  spark  discharge,  the  electric 
oscillation  continues,  and  will  go  on  in  a  succession 
of  sparks,  each  feebler  than  its  predecessor  and 
each  accompanying  a  half-wave  of  radiated  en- 
ergy thrown  off  into  space.  After  a  certain  number 


Fio.  33. — Diagram  of  Electric  Impulses  Delivered  to 

Antenna  by  Induction  Coil. 

of  sparks  have  passed,  depending  upon  the  length 
of  the  gap  and  other  conditions,  the  impulse  re- 
maining is  no  longer  able  to  follow  up  by  another 
spark  and  the  train  of  dwindling  oscillations 
ceases.  Fig.  33  shows  diagrammatically  the 
succession  of  electric  impulses  generated  in  the 
secondary  coil  under  the  action  of  the  vibrator 
V,  or  at  the  rate  which  we  have  assumed  to  be 
200  per  second.  The  electromotive  force  rises 
to  E  with  a  sudden  jump  at  each  interruption  of 
the  vibrator,  and  then  changes  more  slowly  to  a 


THE  SIMPLE  ANTENNA  113 

smaller  magnitude  in  the  opposite  direction. 
The  sudden  kicks  a  E  are  those  which  excite  os- 
cillations. Ten  of  these  kicks  are  indicated  in 
one-twentieth  of  a  second.  Each  impulse  occurs 
in  one  two-hundredth  of  a  second,  which  is 
F  of  a  second  or  5,000  microseconds. 


Discontinuity  oj  Electromagnetic  Flashes  from  a 
Coil-Fed  Antenna 

Fig.  34  indicates  the  surges  or  oscillations  set 
up  in  the  antenna  at  every  kick  or  impulse  a  E 


FIG.  34. — Diagram  of  Oscillations  Set  Up  in  the  Antenna 
at  Each  Electric  Impulse  in  Secondary  Coil. 

in  Fig.  33,  as  set  up  by  the  vibrator  V  of  Fig.  32. 
The  illustrations  show  six  complete  waves  A,  C, 
E,  G,  I,  K  or  twelve  successively  reversed  im- 
pulses of  steadily  diminishing  amplitude.  The 


l\4  WIRELESS  TELEGRAPHY 

last  impulse  L  is  supposed  to  be  too  feeble  to 
create  another  spark  at  the  gap  g  of  Fig.  32,  so 
that  the  series  of  oscillations  comes  to  an  end 
at  L.  Each  complete  or  double  oscillation  is 
represented  as  occupying  one  microsecond,  cor- 
responding to  an  emitted  wave-length  of  300 
meters  (328  yards)  or  a  height  of  simple  rod 
oscillator  equal  to  75  meters  (82  yards).  It  is 
evident  that  the  whole  series  of  six  complete 
oscillations  only  lasts  for  6  microseconds,  and 
since  the  kicks,  or  stimuli,  from  the  induction 
coil,  only  occur  by  assumption  at  intervals  of 
5,000  microseconds,  there  is  evidently  a  long 
interval  of  darkness  and  inactivity  between  the 
little  flashing  intervals  in  which  the  antenna  is 
giving  out  waves  of  invisible  light,  or  long- wave 
polarized  light.  The  period  of  darkness  is  in  this 
instance  832  times  as  long  as  the  period  of  light. 
If  the  key  K  in  Fig.  32  were  held  steadily  down, 
and  the  vibrator  V  were  thus  kept  at  work,  a  dis- 
tant eye  assumed  capable  of  seeing  this  long- 
wave light  would  see  the  antenna  shine  out,  in  a 
certain  unknown  color,  for  6  microseconds  in 
every  5,000,  like  a  flashing  lighthouse  which  sent 
a  beam  over  the  sea  for  6  seconds  every  eighty- 
three  minutes.  Although  the  numerical  values 
here  assumed  may  vary  in  practice  through  a 
considerable  range,  yet  they  are  fairly  represen- 


THE  SIMPLE  ANTENNA  115 

talive,  and  a  wireless  telegraph  sending  antenna 
in  full  activity  is  many  times  more  intermittent 
than  the  longest  period  flashing  light  house  in  the 
world. 

The  observer  with  the  hypothetical  eye  capable 
of  perceiving  the  long  electromagnetic  waves  of 
wireless  telegraphy  would  always  see  the  flashes 
in  the  direction  of,  or  "on  the  true  bearing,"  as  a 
sailor  would  say,  of  the  sending  antenna,  but  the 
flash  would  appear  tangential  to  the  surface  of 
the  ocean,  or  in  the  true  level  horizontal  plane, 
as  distinguished  from  the  actual  visible  horizon 
which  is  depressed  somewhat  below;  so  that  the 
observer  would  expect  to  see  the  luminous  image 
of  the  antenna  thrown  up,  as  though  by  mirage, 
to  the  level  of  his  eye  or  to  the  horizontal  plane 
at  his  level.  Long  after  the  antenna  ceased  to 
be  visible  by  ordinary  short-wave  light,  which 
moves  in  radial  straight  lines,  he  would  expect 
to  see  the  flash  of  the  antenna  by  the  bending  of 
the  long  waves  around  the  conducting  curved 
surface  of  the  sea. 

Nature  and  Use  of  Auxiliary  Condenser  at 
Sending  Antenna 

An  important  piece  of  apparatus  auxiliary 
to  the  antenna  when  set  in  oscillation  is  a 


Ii6  WIRELESS  TELEGRAPHY 

''condenser."  It  consists  of  an  expanded  pair  of 
opposed  conducting  surfaces,  such  as  tin  foil,  sepa- 
rated by  relatively  thin  sheets  or  intervals  of  insu- 
lating material,  such  as  glass,  mica,  oiled  paper, 
oil  or  compressed  air.  A  simple  condenser  may 
be  formed  of  a  sheet  of  window  glass,  coated  on 
each  side  with  tin  foil,  except  near  the  edges. 
The  thinner  the  glass  and  the  larger  its  surface 
area,  the  greater  the  electric  charge  it  will  hold, 
or  the  more  electric  flux  and  electric  flux  energy 
it  will  stow  away  in  the  glass,  for  a  given  magni- 
tude of  charging  voltage,  or  in  technical  lan- 
guage, the  greater  becomes  its  capacity.  Another 
well-known  form  of  condenser  is  a  glass  bottle, 
coated  on  the  inside,  as  well  as  on  the  outside, 
with  tin  foil.  It  is  the  glass  walls  of  this  bottle 
or  Leyden  jar,  which  receive  the  electric  flux  and 
flux  energy  when  the  jar  is  charged.  The  greater 
the  surface  of  the  jar  and  the  thinner  its  wall,  the 
greater  will  be  its  capacity. 

Looking  at  an  antenna  as  a  condenser,  or 
Leyden  jar,  the  surface  area  of  the  conductor  or 
conductors  composing  it  may  be  considerable; 
but  the  slab  of  insulating  air  between  the  antenna 
and  the  ground  is  on  the  average  many  meters 
thick.  Consequently,  the  capacity  of  an  antenna 
is  relatively  small.  An  antenna  50  meters  (54.7 
yards)  high,  even  if  made  up  of  numerous  wires, 


THE  SIMPLE  ANTENNA 


may  have  no  more  capacity  than  a  single  Leyden 
jar  of  ordinary  size. 

A  diagrammatic  view  of  the  electric  flux  stowed 
away  in  the  dielectric  of  a  charged  condenser  is 
shown  in  Fig.  35,  where  the  upper  conducting 
plate  is  represented  as 
being  charged  positively. 
The  flux  density  increas- 
es with  the  thinness  of 
the  insulating  slab  and 
also  with  the  charging 
voltage.  A  limit  to  the 
thinness  of  the  insulator 
is  set,  however,  by  the 
electric  strength  of  the 
material,  which  ruptures, 
or  breaks  down  in 

spark  discharge,  if  a  certain  electric  intensity  is 
exceeded.  Air  at  an  ordinary  pressu/e  and 
temperature  has  a  strength  (between  parallel 
planes)  of  about  4  kilovolts  (4,000  volts)  per 
millimeter  (101,600  volts  per  inch),  glass  8  kilo- 
volts  per  millimeter,  mica  25  kilovolts  per  milli- 
meter, and  so  on  for  other  substances.  The 
electric  strengths  are  affected  by  the  purity  of 
the  material,  its  temperature  and  other  condi- 
tions. 


FIG.  35.  —  Diagrammatic 
Section  of  a  Charged 
Condenser  Formed  by 
Two  Parallel  Plates, 
Showing  the  Distribu- 
tion of  Electric  Flux  in 
the  Insulator  Between 
Them. 


WIRELESS  TELEGRAPHY 


Adjustment  of  Auxiliary  Condenser  Circuit  to 
Consonance  with  Antenna 

A  condenser  is  often  connected  in  parallel  with 
the  antenna  at  the  sending  station  in  the  manner 


CO 

FIG.  36. — Condenser  Connected  in  Parallel  with  Antenna 
to  Re-enforce  Oscillations. 

indicated  in  Fig.  36.  By  this  means  the  capac- 
ity of  the  insulated  system  may  be  much  increased 
so  that  it  will  receive  a  much  greater  electric 
charge  from  the  induction  coil,  with  correspond- 
ingly increased  electric  flux  and  electric  flux 


THE  SIMPLE  ANTENNA  1 19 

energy.  When  the  system  of  Fig.  36  is  dis- 
charged at  the  spark-gap,  the  energy  released  in 
radiation  may  be  considerably  increased,  owing 
to  the  presence  of  the  condenser  and  its  electric 
flux  contents.  On  the  other  hand,  however,  the 
auxiliary  circuit  containing  the  condenser  should 
be  adjusted  to  the  length  of  the  antenna,  in  such 
a  manner  that  the  two  shall  oscillate  together,  or 
in  synchronism.  In  other  words,  the  time  of 
oscillation  of  the  condenser  circuit  A  C  B  G 
should  be  adjusted  to  the  time  of  oscillation  of 
the  antenna;  otherwise,  the  oscillations  of  the 
two  will  mutually  interfere,  and  cancel  each 
other;  so  that  if  the  condenser  circuit  is  not 
tuned  into  synchronism  with  the  antenna,  the 
radiation  into  space  may  be  weakened,  instead 
of  being  enhanced,  by  the  presence  of  the  con- 
denser, in  spite  of  the  greater  stock  of  energy 
available. 

The  tuning  of  the  condenser  circuit  may  be 
accomplished  not  only  by  altering  the  size  of  the 
condenser,  but  also  by  altering  the  length  and 
disposition  of  the  wire  connecting  the  condenser 
to  the  induction  coil.  If  this  wire  be  arranged 
in  a  coil  A  or  B,  Fig.  36,  of  several  turns,  the 
effect  of  the  turns  is  to  increase  the  virtual  length 
of  the  wire  in  rapid  proportion,  because  the 
magnetic  flux  generated  around  any  one  turn 


120  WIRELESS  TELEGRAPHY 

links  also,  more  or  less,  with  the  other  turns. 
By  suitably  adjusting  the  number  of  turns  of 
wire  in  the  condenser  circuit,  the  free  period  of 
oscillation  discharge  of  the  condenser  can  be  ad- 
justed into  synchronism  with  that  of  the  antenna; 
so  that  the  latter  can  thereby  be  thrown  into  re- 
enforced  oscillation  and  wave  emission. 


Loaded  Antennas 

It  is  also  possible  to  alter  the  virtual  length  of 
the  antenna,  by  connecting  a  coil  of  a  few  turns 
of  wire  in  its  circuit  as  indicated  in  Fig.  37.  Such 
an  antenna  is  called  a  loaded  antenna  to  distin- 
guish it  from  the  simple  or  unloaded  antenna  of 
Fig.  26.  An  antenna  loaded  by  a  simple  coil  as 
in  Fig.  37  always  behaves  as  though  its  length 
were  increased.  That  is  its  wave-length  is  in- 
creased, or  its  frequency  of  oscillation  is  reduced. 
Whereas,  therefore,  a  simple  antenna,  say  25 
meters  (27.3  yards)  in  height,  would  throw  off 
waves  100  meters  (109.4  yards)  long,  or  with  a 
frequency  of  3,000,000  cycles  per  second,  or  with 
a  period  of  -J  of  a  microsecond;  the  same  antenna 
loaded  with  a  coil  might  readily  increase  its  wave- 
length to  a  kilometer  or  more  (1094  yards)  with 
a  frequency  of  300,000  cycles  per  second.  At 
the  same  time,  however,  the  radiating  power  of 


THE  SIMPLE  ANTENNA 


121 


the  antenna,  or  the  energy  it  can  throw  off 
in  a  single  wave  is  likely  to  be  greatly  reduced 
by  loading. 

It  is  thus  possible  to  adjust  the  antenna  and 
the  condenser  into  synchronism  by  altering  the 


C±J 

pIG>    37. — Coil    Inserted  in    Series  with  Antenna    to 
Increase  Its  Virtual  Length. 

number  of  turns  of  wire  in  circuit  with  either. 
It  is  also  possible  to  lengthen  the  wave  emitted 
by  an  antenna,  within  certain  limits,  by  loading 
it  with  an  appropriate  coil,  releasing  the  energy 
in  a  longer  train  of  feebler  waves  instead  of  a 
very  short  train  of  stronger  waves. 


122  WIRELESS  TELEGRAPHY 

The  length  of  the  wave  commonly  employed 
in  ordinary  wireless  telegraphy  varies  from  say 
100  meters  to  10  kilometers  (109  yards  to  6.2 
miles)  corresponding  to  frequencies  between 
3,000,000  and  30,000.  A  common  wave-length 
would  be,  say,  300  meters  (328  yards)  with  a  fre- 
quency of  i,ooOyOoo  cycles  per  second. 

It  may  be  observed  that  the  presence  of  the 
induction  coil  I  in  Figs.  32,  36  or  37  does  not 
appreciably  affect  the  virtual  length  of  the 
antenna,  because  it  is  in  parallel  to  the  spark-gap 
G  and  not  in  series  therewith,  like  the  coil  L  of 
Fig.  37.  If  the  induction  coil  I  were  throvn  in 
series,  thereby  adding  its  virtual  length  to  the 
antenna,  the  frequency  would  be  insignificantly 
low.  On  account  of  the  very  large  number  of 
turns  in  the  secondary  coil  I,  or,  as  it  is  technically 
expressed,  on  account  of  its  large  s el] -induction, 
the  oscillations  set  up  on  the  antenna  pass  across 
the  gap  G  and  are  unable  to  find  their  way 
through  the  wire  of  the  coil. 

It  is  also  possible  to  insert  a  condenser  into  the 
.path  of  the  vertical  antenna  with  the  tendency  of 
increasing  the  frequency  of  oscillation,  or  of 
diminishing  the  virtual  height  of  the  antenna. 
The  actions  of  a  condenser  and  a  coil  are  in  this 
respect  opposite  to  each  other* 


THE  SIMPLE  ANTENNA  123 

Danger  o]  Secondary  Internal   Reflections   Oc- 
curring In  a  Loaded  Antenna, 

When,  however,  any  sudden  obstacle  or  appa- 
ratus, such  as  a  ceil,  a  condenser,  a  resistance,  or 
a  discontinuity  of  any  kind  is  inserted  in  the  path 
of  an  antenna,  there  is  a  tendency  to  set  up  reflec- 
tions of  the  oscillations  at  the  discontinuity. 
These  reflections  may  break  up  the  rhythm  and 
diminish  the  amplitude  of  oscillation.  Conse- 
quently, care  has  to  be  taken  so  to  introduce  dis- 
continuities into  an  antenna  as  to  minimize  the 
detrimental  effect  of.  internal  partial  reflection; 
or  the  benefit  gained  by  the  insertion  of  the  dis- 
continuity, as  in  adjusting  the  frequency  to 
resonance,  may  be  more  than  offset  by  the 
shattering  of  main  oscillations  into  minor  and 
discordant  ripple  trains. 


CHAPTER   X 

ELECTROMAGNETIC   WAVE-DETECTORS,   OR  WIRE- 
LESS  TELEGRAPH   RECEIVERS 

Voltage  Detectors  and  Current  Detectors 

SINCE  the  human  eye  is  incapable  of  respond- 
ing to  the  long-wave  flashes,  or  electromagnetic 
waves,  given  off  by  a  wireless  telegraph  antenna, 
an  artificially  constructed  eye  has  to  be  used  in 
order  to  detect  and  respond  to  them.  The  plan 
followed  is  to  place  an  antenna  at  any  suitable 
place  in  the  path  of  the  waves,  so  that  oscillating 
electric  impulses  may  be  set  up  in  this  antenna, 
and  then  to  permit  these  impulses  to  act  upon 
some  electromagnetic  apparatus  connected  either 
directly  in  the  path  of  the  antenna,  or  indirectly, 
by  the  aid  of  a  little  induction  coil.  The  receiv- 
ing instrument  must  therefore  be  affected  either 
by  the  oscillatory  voltage,  or  by  the  oscillatory 
current  in  the  antenna. 

A  voltage  detector  may  be  theoretically  any 
apparatus  which  responds  to  electric  potential 
difference;  such  as  an  electroscope  or  a  pair  of 
124 


ELECTROMAGNETIC  WAVE-DETECTORS      125 

diverging  gold-leaves.     In  practice,  however,  it 
consists  of  a  little  instrument  called  a  coherer. 

A  current  detector  may  be  of  any  of  the  various 
types  which  are  used  to  indicate  the  presence  of 
high-frequency  alternating  currents.  There  are 
a  number  of  different  receivers  of  which  we  need 
only  consider  the  prominent  types.  In  practice, 
there  are  three  well-known  types:  namely,  the 
thermal,  the  electrolytic  and  the  electromagnetic. 

Coherers 

Coherers  are  illustrated  typically  in  Figs.  38, 
39  and  40.  In  Fig.  38,  we  have  a  sealed  glass 
tube  T  T  about  4  cms.  (i|  inches)  long  and  of 
2j  mm.  (-fa  inch)  bore.  Near  the  middle  of 
the  tube  are  two  metallic  plugs,  P  P,  often  made 
of  silver.  These  are  connected  to  the  external 
wires  WW  by  sealed-in  platinum  connections. 
The  plugs  P  P  do  not  touch,  but  are  separated 
by  a  small  gap  about  J  mm.  (-^-th  inch)  wide. 
The  tube  may  be  partially  exhausted  of  air;  but 
the  gap  between  the  plugs  P  P  contains  fine 
metallic  powder,  or  metallic  dust,  which  lies 
loosely  in  the  little  crevasse. 

The  loose  metallic  particles  bridging  across 
between  the  plugs  P  P  have  the  property  of  offer- 
ing an  obstruction,  or  very  high  resistance,  to  the 
flow  of  current  from  a  single  voltaic  cell.  In 


126 


WIRELESS  TELEGRAPHY 


other  words,  the  gap  is  almost  an  insulator  to 
this  feeble  voltaic  electromotive  force.  If,  how- 
ever, a  higher  electromotive  force  be  applied 
across  the  gap  for  even  a  very  minute  interval  of 
time,  its  effect  is  to  break  down  the  insulator  and 


FIG,  38. 


FIG.  41. 
FIGS.  38,  40  and  41. — Types  of  Coherers. 

to  allow  the  voltaic  cell  to  send  a  continuous 
current.  The  sudden  higher  electric  impulse 
changes  the  resistance  offered  by  the  bridge  of 
metallic  dust  from  a  very  high  to  a  relatively  low 
value. 

The  exact  nature  of  the  action  which  takes 
place  when  the  electric  impulse  operates,  and 


ELECTROMAGNETIC  WAVE-DETECTORS     127 

when  the  resistance  of  the  gap  breaks  down,  is 
hard  to  determine  with  certainty.  It  has  been 
much  discussed  and  unanimity  has  not  yet  been 
reached  upon  the  matter.  It  suffices  for  present 
purposes,  however,  to  say  that  the  extra  voltage 
brought  to  bear  across  the  gap  of  metallic  par- 
ticles causes  them  to  weld  together,  or  to  cohere 
electrically,  thus  converting  a  very  bad  joint  in 
the  local  circuit  of  the  voltaic  cell  into  a  fairly 
good  one. 

Connections  Between  Coherer  and  Antenna 

The  simplest  method  of  connecting  the  coherer 
of  Fig.  38  with  the  receiving  antenna  is  indicated 
in  Fig.  39.  A  B  C  S  G  is  the  antenna  path  to 
ground.  It  is  cut  at  C,  and  the  coherer  inserted 
by  means  of  the  wires  W  W,  Fig.  38.  A  local 
circuit  EMC,  indicated  in  broken  lines,  con- 
nects a  suitable  low  voltaic  electromotive  force, 
such  as  a  single  voltaic  cell,  to  the  coherer  ter- 
minals through  an  electromagnetic  receiver,  rep- 
resented as  an  ordinary  wire-telegraph  sounder. 
Prior  to  the  arrival  of  electromagnetic  waves,  the 
gap  of  filings  in  the  coherer  interposes  a  high 
resistance  in  the  local  circuit,  as  well  as  in  the 
antenna  path.  Consequently,  no  appreciable 
current  flows  through  the  sounder  M,  the  arma- 
ture lever  of  which  remains  released  against  its 


128 


WIRELESS  TELEGRAPHY 


upper  stop  under  the  action  of  a  spiral  spring. 
As  soon  as  an  electromagnetic  wave,  or  wave- 
train,  of  suitable  intensity  passes  the  antenna,  an 
oscillating  electromotive  force  will  be  set  up  along 
the  antenna  and  across  the  coherer  gap.  The 


FIG.  39. — Essential  Elements  of  Coherer-  Connections 
When  Receiving  Signals. 

coherer  will  instantly  break  down  in  insulation, 
and  will  cause  the  metallic  filings  to  cohere.  The 
oscillations  will  not  discharge  through  tne  local 
circuit  owing  to  the  self-induction  or  choking 
effect  of  the  coil  on  the  magnet  M.  The  voltaic 
cell  E  will  now  be  able  to  send  a  current  through 


ELECTROMAGNETIC  WAVE-DETECTORS     129 

the  local  circuit  EMC  and  excite  the  electro- 
magnet M  of  the  sounder,  the  armature  lever  of 
which  will  descend  with  a  click,  thus  giving  evi- 
dence of  the  arrival  of  the  wave. 

Mechanical  Decoherence 

The  current  in  the  local  circuit  would  continue 
indefinitely  after  the  bridging  of  the  coherer  gap 
by  the  first  wave,  if  means  were  not  provided  for 
decohering,  or  restoring  the  coherer  to  its  original 
insulating  state.  This  may  be  done  by  giving  a 
tap  or  light  mechanical  agitation  to  the  coherer 
tube,  thus  shaking  up  the  filings  in  the  gap  and 
breaking  up  the  recently  welded  bridge  between 
the  plugs  P  P,  Fig.  38.  In  practice,  the  arma- 
ture lever  of  the  sounder  M,  may  be  arranged  to 
deliver  a  light  tap  to  the  coherer  tube  at  the 
same  moment  that  it  produces  its  click.  This 
tap  restores  the  original  condition  of  the  coherer, 
interrupts  the  local  circuit  and  cuts  off  the  excita- 
tion from  the  sounder  magnet  M,  which  promptly 
releases  its  armature  lever  under  the  influence  of 
the  spiral  spring;  so  that  the  apparatus  is  again 
ready  to  respond  to  the  next  electromagnetic 
wave. 

The  connections  of  the  local  circuit  are  usually 
somewhat  more  complex  than  Fig.  39  "shows; 
but  the  principle  remains  essentially  the  same. 


130  WIRELESS  TELEGRAPHY 

The  sounder  M,  for  instance,  is  not  directly 
actuated  by  the  local  circuit  of  the  coherer,  be- 
cause it  needs  a  relatively  strong  current,  which 
is  unsuitable.  A  delicate  electromagnet  called 
a  relay  (see  Figs.  60  and  61)  is,  however,  placed 
in  the  circuit  at  the  point  M,  and  the  armature 
lever  of  the  relay  closes  another  local  circuit 
through  a  more  powerful  voltaic  battery  and  the 
sounder.  A  feeble  current  through  the  local 
circuit  of  the  coherer  is  thus  enabled  to  send  a 
suitably  strong  current  through  the  sounder.  An 
auxiliary  electromagnet,  actuated  also  by  the 
relay,  is  often  applied  to  the  sole  duty  of  tapping 
the  coherer,  or  causing  it  to  decohere,  after  the 
relay  and  sounder  have  responded. 

It  is  evident  that  when  the  apparatus  is  in 
working  order,  signals  consisting  of  short  and 
long  groups  of  electromagnetic  waves  will  be  able 
to  spell  out  corresponding  short  and  long  opera- 
tions of  the  electromagnet  M,  or  dots  and  dashes 
of  the  Morse  alphabet. 

Fig.  40  represents  a  modified  form  of  coherer 
in  which  there  are  two  gaps  g  g.  In  each  gap 
there  is  a  little  globule  of  mercury.  The  end 
plugs  C  C  may  be  of  carbon,  and  the  central  plug 
a  little  cylinder  of  iron.  This  form  of  coherer 
has  the  advantage  that  it  is  self-decohering,  or 
auto-decohering.  That  is  to  say  it  needs  no 


ELECTROMAGNETIC  WAVE-DETECTORS     131 

blow  or  agitation  to  restore  the  status  quo  after 
the  passage  of  a  wave.  It  normally  possesses  a 
high  resistance.  The  electric  oscillation  or  surge 
in  the  antenna  breaks  down  this  resistance 
momentarily  and  permits  a  current  to  flow 
through  a  local  voltaic  circuit.  Immediately  af- 
ter the  passage  of  the  wave  the  high  internal 
resistance  is  restored.  The  reason  for  this  re- 
markable action  is  concealed  in  the  general 
obscurity  of  the  whole  subject  of  coherence,  but 
is  perhaps  connected  with  the  liquid  state  of  the 
substance  in  the  gaps. 

Another  form  of  coherer  is  indicated  in  Fig.  41. 
It  consists  of  a  small  insulating  vessel  or  reservoir 
V,  containing  mercury.  The  mercury  is  brought 
into  very  light  contact  with  the  thin  edge  of  a 
metallic  disk  P,  kept  rotating  by  clockwork. 
There  is  a  very  thin  film  of  insulating  oil  on  the 
surface  of  the  mercury  and  the  effect  of  the  film 
is  to  insulate,  or  electrically  separate,  the  metallic 
disk  from  the  mercury.  The  thin  film,  may, 
however,  be  broken  down,  or  electrically  dis- 
rupted, by  a  relatively  feeble  voltage  in  excess  of 
that  used  in  a  local  voltaic  circuit.  The  wires 
W  W  connect  the  device  with  the  local  circuit 
and  the  antenna,  as  in  Fig.  39.  On  the  arrival 
of  a  signal,  theT)il  film  is  broken  and  contact  es- 
tablished between  the  disk  and  mercury;  but 


132 


WIRELESS  TELEGRAPHY 


the  revolution  of  the  disk  almost  instantly  re- 
stores the  oil  film  and  decoheres  the  device. 

Hot-wire  Receivers 

Coherers  depend,  as  we  have  seen,  upon  the 
electromotive  force  or  voltage  of  the  surge  set  up 
in  the  antenna  to  force  a  discharge  across  the  gap 
of  imperfectly  contacting 
matter,  in  order  to  give 
passage  to  a  local  vol- 
taic current.  Among  re- 
ceivers which  depend, 
however,  upon  the  mag- 
nitude of  oscillating  cur- 
rent set  up  through  them 
when  they  are  inserted  in 
the  path  of  the  receiv- 
ing antenna,  without  interrupting  the  same,  we 
have  the  hot-wire  receiver.  One  form  of  this 
device  is  presented  to  view  in  Fig.  42.  A  pair 
of  parallel  brass  strips  AB,  CD,  are  fastened 
near  to  each  other  and  side  by  side,  by  an  insulat- 
ing block  F  of  hard  rubber.  Leading-in  wires 
W  W  are  soldered  to  these  strips  above,  at  A  and 
C.  Between  the  lower  adjacent  corners  is  sol- 
dered a  little  piece  of  silver  wire  e,  bent  into  the 
form  of  a  sharp  V.  This  silver  wire  may  be 
about  3  millimeters  (0.12  inch)  long  and  about 


FIG.  42.—  Hot-Wire 
Receiver, 


ELECTROMAGNETIC   WAVE-DETECTORS     133 

0.076  millimeter   (0.003  inch)   in  diameter.     A 

cross  section  of  this  silver  wire  is  indicated  in 

Fig.  43,  at  A  B  C.     At  or  near  the  center  q  is  a 

thin  filamentary  wire  of 

platinum,  like  the  wick 

inside  a  paraffin  candle. 

The    diameter   of    the  FIG.  43. — Cross -Section  of 

,     .  .   ,     .        ,  Composite  Wire. 

platinum  wick  is  about 
i.  5  microns  (0.0015  milli- 
meter or  0.000,06  inch), 
or  about  one-fortieth  of 
the  diameter  of  a  thin  hu- 
man hair.  A  platinum 
wire  so  fine  is  only  ob- 
tained by  thickly  coating 
an  ordinary  size  of  plat- 
inum wire  with  silver,  FlG  44._view  of  Loop  of 

and  then  drawing  down       Sensitive  Fine  Wire  tin- 
der Microscope, 
the  thick  composite  wire 

through  successively  diminishing  dies.  As  the 
silver  wire  gets  thinner  and  longer,  so  also  does 
the  internally  held  wick  or  filament  of  platinum. 
After  the  little  V  loop  of  silver  candle-wire  has 
been  soldered  to  the  brass  plates  at  B  and  D, 
Fig.  42,  the  device  is  carefully  lowered  into  a 
bath  of  nitric  acid,  in  such  a  manner  that  the 
point  of  the  V  loop  is  submerged  in  the  acid, 
which  immediately  attacks  and  dissolves  the  sil- 


134  WIRELESS  TELEGRAPHY 

ver  chemically,  leaving  the  platinum  wick  un- 
injured. The  process  is  aided  by  a  feeble  electric 
current  from  a  local  voltaic  cell,  is  watched  under 
the  microscope,  and  is  arrested  at  the  proper 
stage.  The  appearance  in  the  microscope  of  the 
V  loop  after  the  silver  has  been  dissolved  off  the 
tip  is  shown  in  Fig.  44,  where  A  B  and  C  D  are 
the  76-micron  or  0.076  millimeter  silver  wires, 
and  e  f  g  the  1.5  micron  platinum  filament, 
hanging  in  a  short  loop.  The  device  is  then  ready 
for  use  and  is  conveniently  protected  from  injury 
by  placing  it  in  a  short  glass  bottle  or  test-tube. 

The  connection  of  the  little  hot-wire  device 
with  the  receiving  antenna  is  illustrated  in  its 
simplest  elements  at  Fig.  45.  A  B  is  the  antenna, 
connected  to  ground  through  the  hot-wire  at  H. 
A  local  voltaic  circuit,  in  broken  lines,  connects 
a  feeble  electromotive  force,  such  as  a  single 
voltaic  cell,  through  the  telephone  receiver  T  and 
the  hot-wire  H.  Prior  to  the  advent  of  electro- 
magnetic waves,  a  steady  current  flows  through 
the  local  voltaic  circuit,  producing  no  sound  in 
the  receiver  T.  This  current  serves  to  warm  the 
fine  platinum  wire,  the  electric  resistance  of 
which  is  appreciable,  but  constant  at  any  con- 
stant temperature.  As  the  temperature  of  the 
platinum  is  increased,  however,  the  resistance 
increases. 


ELECTROMAGNETIC  WAVE-DETECTORS     135 

If  now  an  electromagnetic  wave  or  wave-train 
strikes  the  antenna  B  A,  an  oscillating  current 
will  pass  through  the  fine  platinum  filament  H, 
and  will  heat  the  same  appreciably,  being  super- 
posed upon  the  steady  current  from  the  voltaic 


FIG.  45. — Connection  of  Hot- Wire  Receiver  with 
Receiving  Antenna. 

cell  E.  The  antenna  is  prevented  from  dis- 
charging to  ground  through  the  telephone  T; 
or  by  the  path  AB  TE  SG,  owing  to  the  self- 
induction,  or  choking  effect,  of  the  telephone. 
Practically  all  the  discharge  goes  through  H. 
The  momentary  increase  in  the  heat  and  tern- 


136  WIRELESS  TELEGRAPHY 

perature  of  the  filament  H  causes  its  resistance 
to  be  momentarily  raised,  and  this  reacts  upon 
the  local  voltaic  current,  diminishing  the  same 
momentarily.  The  telephone  T  responds  audi- 
bly to  the  sudden  alteration  of  current,  which 
lasts  as  long  as  the  waves  or  groups  of  waves  are 
passing,  and  ceases  the  moment  the  waves  cease 
to  arrive.  The  sensitiveness  of  the  device  is  due 
to  the  small  dimensions  of  the  fine  filament.  The 
oscillating  electric  currents  received  through  the 
filament  from  the  antenna  are  very  feeble  when 
the  antenna  is  far  from  the  sending  station;  but 
the  cross- section  of  the  filament  being  only  about 
2  square  microns  (-g-grj-  of  one  millionth  of  a 
square  inch),  even  a  very  feeble  electric  current 
will  be  condensed  to  a  relatively  appreciable  cur- 
rent density  at  the  filament,  thus  giving  rise  to 
appreciable  heating  in  a  mass  of  metal  only 
about  2,000  cubic  microns  in  volume  (^-sV^-th  of 
one  millionth  of  a  cubic  inch). 

Electrolytic  Receivers 

The  device,  represented  in  outline  by  Fig.  46, 
consists  of  a  small  vessel  C  containing  a  suitable 
solution,  such  as  dilute  nitric  acid.  A  candle 
wire  w,  of  the  kind  above  described  in  connection 
with  Figs.  43  and  44;  i.e.,  a  silver  wire  of  about 
76  microns  in  diameter  (0.003  inch)  with  a 


ELECTROMAGNETIC  WAVE-DETECTORS     137 

platinum  wick  about  1.5  microns  in  diameter 
(0.000,06  inch),  is  immersed  to  a  suitable  depth, 
perhaps  a  quarter  millimeter  (o.oi  inch)  in  the 
solution.  The  acid  dissolves  off  the  silver,  so 
that  the  filament  of  plat- 
inum is  immersed  in  the 
solution,  offering  thereto 
an  immersed  surface  area 
of  about  1,200  square 
microns  (2,660,060th  of  a 
square  inch).  The  wire 
w  is  fastened  to  the  lower 
end  of  a  brass  screw  hav- 
ing a  milled  head  ss,  the 
screw  passing  through  a 
brass  support  P  P.  The 
depth  of  immersion  of  the 
fine  platinum  filament  can  be  adjusted  by  turn- 
ing the  milled  head  in  one  or  the  other  direction. 

A  solution  of  an  acid  or  alkali  traversed  by  an 
electric  current  is  called  an  electrolyte,  and  the 
current  is  carried  only  by  atoms,  or  groups  of 
atoms,  which  are  separated  out  from  the  solu- 
tion. In  other  words  electric  conduction  through 
an  electrolyte  is  accompanied  by  chemical  de- 
composition of  the  electrolyte. 

The  connection  of  the  electrolytic  receiver  with 
the  receiving  antenna  is  essentially  the  same  as 


FIG.  46. — Electrolytic 
Receiver. 


138  WIRELESS  TELEGRAPHY 

that  of  the  hot-wire  receiver  represented  in  Fig. 
45.  That  is,  a  local  voltaic  current  is  provided 
containing  a  small  electromotive  force  and  a 
telephone  receiver.  Prior  to  the  advent  of  the 
electromagnetic  waves,  the  cell  E,  Fig.  45,  sends 
a  feeble  steady  current  through  the  telephone 
and  the  electrolytic  receiver.  This  current 
causes  minute  bubbles  of  gas  to  be  liberated  from 
the  fine  immersed  platinum  filament,  but  the 
telephone  T  gives  no  sound.  As  soon  as  an 
electromagnetic  wave  strikes  the  antenna,  the 
oscillating  current  set  up  passes  through  the 
electrolytic  receiver,  and  heats  the  same  in  the 
minute  constricted  mass  of  liquid  immediately 
surrounding  the  fine  platinum  filament.  The 
effect  of  the  heat  so  liberated  is  two-fold.  In  the 
first  place  it  momentarily  raises  the  temperature 
of  the  pellicle  of  solution  immediately  surround- 
ing the  filament,  thereby  reducing  the  electric 
resistance  of  the  device;  for  electrolytes,  unlike 
metals,  improve  in  conductivity  when  heated.  In 
the  second  place  the  gas  is  liberated  more  freely 
from  the  surface  of  the  fine  filament  as  the  tem- 
perature is  increased.  In  technical  language, 
the  momentary  warming  effect  of  the  oscillating 
current  causes  the  filament  to  be  partly  depolar- 
ized. Owing  to  both  of  these  actions,  the  appar- 
ent resistance  offered  to  the  local  current  from 


ELECTROMAGNETIC  WAVE-DETECTORS    139 

the  voltaic  cell  is  temporarily  diminished  and  the 
sudden  increase  of  current  produces  a  sound  in 
the  telephone.  Immediately  after  the  passage  of 
the  wave,  the  heat  is  dissipated  by  conduction 
into  the  solution,  and  the  original  resistance  and 
counter  electromotive  jorce  of  polarization  in  the 
device  are  restored. 

The  sensitiveness  of  this  device  is  attributable 
to  the  constriction  of  the  conducting  path  from 
antenna  to  ground  into  a  minute  volume  of 
liquid,  having  an  'extremely  small  cross-section. 
In  this  tiny  volume  there  exists  a  very  appreciable 
electric  resistance  and  also  an  appreciable  elec- 
trolytic back  voltage,  like  that  of  an  opposing 
voltaic  cell.  The  thermal  effects  of  even  a  very 
feeble  oscillating  current  from  the  antenna  are 
here  condensed  into  so  small  a  volume  that  the 
temperature  of  that  small  volume  can  be  appre- 
ciably raised.  The  rise  of  temperature  has  a 
powerful  effect  both  on  the  localized  resistance 
of  the  constricted  liquid  path  and  on  the  back 
voltage  of  the  virtual  opposing  voltaic  cell  con- 
tained in  the  device. 

Another  form  of  the  electrolytic  receiver  is 
represented  in  Fig.  47.  A  vessel,  V  V  V,  such 
as  a  small  glass  tumbler,  is  filled  with  an  elec- 
trolyte such  as  dilute  nitric  acid,  or  dilute  caustic 
soda,  to  the  level  L  L.  Two  metallic  surfaces 


140 


WIRELESS  TELEGRAPHY 


or  electrodes  dip  into  the  electrolyte.  One  of 
these,  indicated  at  E,  may  have  any  convenient 
size  and  shape,  connecting  with  a  leading-in 
wire  W2;  or  this  wire  may  itself  dip  into  the 
solution  and  form  the  electrode  E.  The  other 


FIG.  47. — Simple  Form  of  Electrolytic  Receiver. 

electrode  like  w,  in  Fig.  46,  has  extremely  small 
surface  area.  A  very  simple  way  of  preparing 
such  a  small  area  of  electrode  is  illustrated  in 
greater  detail  at  the  lower  part  of  the  Figure.  A 
glass  tube,  a  b  c  of  any  convenient  dimensions, 
say  7.5  cms.  (3  inches)  long,  3  mm.  and  i  mm. 
(about  J  and  -fa  inch)  in  external  and  internal 


ELECTROMAGNETIC  WAVE-DETECTORS     141 

diameters  respectively,  is  slipped  over  a  short 
length  of  copper  wire.  This  copper  wire  is 
welded  at  one  end  in  the  flame  of  a  Bunsen 
burner  to  a  few  centimeters  (one  inch,  say)  of 
fine  platinum  wire  having  a  diameter  of  about 
0.05  mm.  (0.002  inch).  In  the  illustration,  the 
weld  or  junction  between  the  copper  and  plati- 
num wires  is  shown  at  J.  The  tube  is  then 
heated  to  the  softening  point  over  the  fine  plati- 
num wire,  and  the  softened  walls  are  squeezed 
tightly  over  the  platinum  wire  with  a  small  pair 
of  tongs,  so  as  to  seal  in  the  wire  hermetically  for 
a  short  distance,  say  i  cm.  (f  inch).  The  tube 
after  cooling  is  now  scratched  with  a  file  across 
the  seal  and  is  broken  sharply  across  with  the 
fingers.  The  break  if  properly  made  will  leave 
the  ruptured  platinum  wire  very  slightly  project- 
ing beyond  the  glass.  A  few  strokes,  with  a  fine 
file,  will  file  the  platinum  wire  flush  with  the 
glass,  thus  presenting  as  at  d,  Fig.  47,  an  exposed 
disk  of  platinum  of  the  diameter  of  the  fine  wire 
at  the  end  of  the  seal  b  c,  and  with  a  surface  area 
of  about  2,000  square  microns  or  T,7rdr,innr  square 
inch. 

The  depth  of  immersion  of  the  glass  tube  in 
the  electrolyte  is  of  no  consequence  so  long  as  the 
end  of  the  fine  platinum  wire  is  fairly  covered  by 
the  solution.  Neither  does  the  distance  between 


142  WIRELESS  TELEGRAPHY 

the  two  electrodes  in  the  vessel  V  V,  make  any 
appreciable  difference.  In  other  words,  the  seat 
of  the  actions  occurring  in  the  apparatus  is  in 
the  constricted  liquid  path  immediately  covering 
and  including  the  minute  exposed  area.  The 
resistance  of  the  device  is  practically  all  located 
within  a  millimeter  (^  inch)  of  the  exposure,  and 
the  voltaic  counter  electromotive  force  of  polar- 
ization is  located  at  the  exposure;  so  that  all  the 
rest  of  the  electrolyte  merely  provides  an  out- 
ward escape  for  the  electric  current  that  has 
passed  through  the  intensely  constricted  region 
of  influence  over  the  minute  area. 

This  form  of  electrolytic  receiver  is  probably 
the  simplest  to  construct  of  all  the  receivers  used 
in  wireless  telegraphy.  Like  most  of  the  other 
devices  it  is  patented.  It  is  not  capable  of  ad- 
justment in  surface  exposure  area,  like  the  appa- 
ratus of  Fig.  46,  and  if  the  minute  exposure  gets 
dirty  or  clogged,  it  has  to  be  thrown  away  and  a 
new  one  substituted.  It  is,  however,  capable  of 
great  sensitiveness  and  the  materials  for  its  con- 
struction are  very  easily  obtained. 

Electromagnetic  Receivers 

There  are  various  types  of  electromagnetic 
receivers,  but  that  illustrated  in  Fig.  48  is  gener- 
ally admitted  to  be  the  most  sensitive.  It  con- 


ELECTROMAGNETIC  WAVE-DETECTORS     143 

sists  of  a  flexible  band  b  b  b  b  of  iron  wires  pass- 
ing over  the  grooved  pulleys  L  L,  which  are 
steadily  driven  by  clockwork.  The  band  of  iron 
wires  moves  through  a  glass  tube  1 1,  on  which  is 
placed  a  winding  of  insulated  wire  with  external 
connections  W  W,  leading  to  the  antenna  on  one 
side  and  to  ground  on  the  other.  This  winding, 


T 

FIG.  48. — Magnetic  Receiver. 

inserted  in  the  antenna  path,  forms  the  primary 
winding  of  a  little  induction  coil,  in  which  the 
moving  band  of  iron  wires  is  the  core.  The 
secondary  winding  S,  placed  over  the  middle  of 
the  glass  tube  is  connected  to  a  telephone  re- 
ceiver, T. 

The  band  of  iron  wires  in  passing  through  the 
tube  makes  its  procession  in  front  of  the  two 
fixed  permanent  horseshoe  magnets  M  M.  These 


144  WIRELESS  TELEGRAPHY 

are  so  arranged  with  regard  to  strength  and 
direction  of  polarity,  that  the  band  of  iron 
emerges  from  the  tube  with  its  internal  magnet, 
ism  reversed  in  direction  from  that  with  which 
it  enters. 

Iron  has  a  curious  magnetic  property  when  its 
magnetism  is  cyclically  reversed.  If  the  mag- 
netism is  established  along  a  band  of  iron  wires 
in  one  direction,  then  when  the  process  of  de- 
magnetization and  reversal  is  started,  the  change 
of  magnetic  flux  in  the  iron  takes  place  very 
slowly  at  first,  until  a  certain  stage  of  magnetic 
instability  is  reached,  and  then  the  magnetic  flux 
reverses  with  great  swiftness.  The  action  may 
be  compared  to  that  of  a  ball  moving  alternately 
from  side  to  side  on  the  deck  of  a  rolling  ship  at 
sea.  If  the  deck  is  flat  and  plane,  the  ball  will 
swing  regularly  from  side  to  side.  If,  however, 
the  deck  be  somewhat  bowed,  rising  in  the  middle 
like  a  turtle-back,  the  ball  will  be  slow  to  return 
on  each  roll  until  it  gets  to  the  top  of  the  turtle- 
back  and  then  it  will  run  down  with  great  speed. 

The  function  of  the  two  magnets  M  M  in  the 
magnetic  receiver  is  to  bring  the  iron  wire  core 
of  the  induction  coil  into  the  unstable  magnetic 
condition  during  its  passage  within  the  primary 
winding  connected  in  the  antenna  path.  Under 
these  conditions,  if  any  electric  oscillations  come 


ELECTROMAGNETIC  WAVE-DETECTORS     145 

through  the  primary  coil  from  the  antenna,  they 
will  be  able  to  shake  out  the  magnetic  flux  in  the 
enclosed  band  and  precipitate  its  reversal.  The 
rotating  mechanism  brings  the  magnetic  flux  to 
the  edge  of  the  precipice,  as  it  were,  and  the 
feeble  electric  currents  are  able  to  push  it  over. 
The  sudden  change  of  magnetic  flux  inside  the 
secondary  coil  s,  sets  up  an  electric  impulse  that 
will  produce  an  audible  sound  in  the  telephone  T. 
The  above  form  of  magnetic  receiver  is  thus 
essentially  an  induction  coil  with  the  antenna 
path  passing  through  the  primary  winding  and 
the  delicate  receiving  telephone  in  the  secondary. 
The  induction  is  increased  in  sensitiveness  by  the 
aid  of  the  constantly  renewed  magnetic  instabil- 
ity in  the  iron  core,  under  the  action  of  the  per- 
manent magnets. 

Comparison  0}  Receivers 

Comparing  the  behavior  of  the  various  types 
of  receiver,  it  is  to  be  noted  that  the  coherer  is 
the  only  one  which  permits  of  a  permanent  record 
being  obtained.  The  coherer,  as  outlined  in 
Fig.  39,  operates  an  electromagnetic  receiver  of 
the  Morse  type.  Such  a  receiver  is  able  to  record 
the  message  in  dots  and  dashes  inked  upon  the 
surface  of  a  long  strip  of  paper,  coiled  on  a  roller 
in  the  apparatus  and  moved  by  clockwork,  A 


146  WIRELESS  TELEGRAPHY 

particular  form  of  Morse  inkwriter  is  seen  in 
Fig.  49.  It  will  be  observed  that  the  armature 
consists  of  a  split  soft  iron  tube  fastened  to  a 
rocking  lever  in  such  a  manner  as  to  be  attracted 
downward  when  the  black  coated  electromagnet 


FIG.  49. — Morse  Inkwriter. 

is  excited.  The  lever  throws  up  a  disk  against  a 
moving  band  of  paper  not  shown.  The  disk  is 
kept  rotating  by  clockwork  and  dips  into  an 
inkwell. 

On  the  other  hand,  however,  the  speed  at 
which  signals  can  be  received  and  recorded  by 
means  of  the  coherer  is  distinctly  lower  than  that 
obtainable  with  non  -  recording  receivers.  A 
speed  of  15  to  20  words  a  minute  is  considered 
good  with  a  recording  receiver.  With  some  non- 
recording  receivers  this  speed  may  be  doubled. 


ELECTROMAGNETIC  WAVE-DETECTORS     147 

In  regard  to  sensitiveness,  the  coherer  has 
hitherto  proved  much  inferior  to  the  others.  The 
most  sensitive  is  the  electrolytic  receiver  and  next 
to  that  the  magnetic.  Both  of  these  use  the 
telephone  as  the  receiving  instrument. 

Telephone  Receivers 

A  convenient  form  of  telephone  receiver,  illus- 
trated by  Fig.  50,  is  such  as  telephone  operators 
employ.  A  leather-covered  steel  band  L  L  goes 
over  the  head  and  supports  the  receiver  R  R  close 
to  one  ear.  The  band  is 
fastened  to  the  receiver  by 
the  thumb-screw  s.  The 
covered  wires  w  w  serve  to 
connect  the  receiver  with 
the  antenna  system. 

Fig.  51  shows  the  parts 
of  the  receiver  disassem- 
bled. B  B  is  a  hard  rub- 
be  r  box  with  a  screw 
cover  C.  Inside  the  box  are  three  pairs  of  half- 
ring  steel  permanent  magnets,  NS,  NS.  In  the 
center,  a  pair  of  soft  iron  pole  pieces  are  sup- 
ported, receiving  their  polarity  from  the  magnets 
NS  and  wound  with  many  turns  of  fine  silk- 
covered  copper  wire  connecting  with  the  leading 
wires  ww.  D  is  the  ferro-type  disk  of  steel 


FIG. 


50.— Head  Tele- 
phone. 


148  WIRELESS  TELEGRAPHY 

which  is  clamped  around  its  edge  between  the 
box  and  the  cover,  so  as  to  be  held  over  but  not 
quite  touching  the  poles  at  the  center. 

The  sensitiveness  of  the  electrolytic  and  mag- 
netic receivers  is  at  least  partly  attributable  to 


FIG.  51. — Head  Telephone  Disassembled. 

the  great  sensitiveness  of  the  telephone  which 
they  employ  as  their  intermediary  with  the 
human  brain.  The  telephone,  as  is  well  known, 
is  extraordinarily  sensitive  in  detecting  feeble 
electric  currents  undergoing  rapid  variations 


CHAPTER   XI 

WIRELESS   TELEGRAPH   WORKING 

Alternate  Use  o]  an  Antenna  jor  Sending  and 
Receiving 

IN  order  to  carry  on  simple  wireless  telegraphy 
between  a  single  pair  of  stations,  remote  from  all 
other  wireless  telegraphists,  it  is  evidently  neces- 
sary to  have  an  antenna  at  each  station.  The 
dimensions  required  for  the  antennas  will  depend 
upon  the  distance  between  them.  For  sending 
messages  from  one  room  to  another  in  the  same 
building,  the  antennas  may  be  a  few  centimeters 
or  inches  long.  For  sending  messages  between 
adjacent  buildings,  or  buildings  separated  only 
by  a  few  kilometers  or  miles,  the  antennas  need 
only  be  a  few  meters  or  yards  high.  For  dis- 
tances of  hundreds  of  kilometers  or  miles,  large 
and  tall  antennas  are  at  present  necessary.  The 
object  of  the  sending  station  in  long-distance 
wireless  telegraphy  is  to  throw  out  as  long  a  train 
of  powerful  waves  as  possible,  while  that  of  the 
receiving  station  is  to  employ  as  sensitive  a  re- 
ceiver as  possible. 

149 


150 


WIRELESS  TELEGRAPHS 


One  and  the  same  antenna  is  used  at  a  station 
for  sending  and  receiving  alternately.  The  con- 
nection is  changed  from  the  sending  to  the  receiv- 
ing apparatus  by  a  switch  as  indicated  in  Fig.  52. 


tife 


FIG.  52. — Diagram  of  Switch  Connections  from  Sending 
to  Receiving. 


The  switch  S,  has  a  metallic  blade  or  lever-arm 
which  is  pivoted  at  c  and  may  be  turned  into  con- 
tact with  the  point  d  for  sending,  or  with  the 
point  r  for  receiving.  A  is  the  antenna,  or  the 
wire  leading  thereto,  and  G  the  ground-connec- 
tion. In  the  position  shown,  the  switch  is  turned 
to  the  sending  position  and  the  antenna  is  con- 
nected to  the  spark-gap  k.  This  is  excited  by 
the  induction-coil  I  and  vibrator  V,  when  the 


WIRELESS  TELEGRAPH  WORKING-      151 

key  K  in  the  primary  circuit  is  depressed.  There 
is  also  an  auxiliary  circuit  X,  consisting  in  this 
case  of  a  pair  of  condensers  with  a  coil  between 
them,  to  increase  the  stock  of  radiation-energy 
prior  to  discharge.  The  receiving  apparatus  is 
indicated  as  consisting  of  a  coherer  H,  working 
into  a  relay  R,  (see  Fig.  61)  through  the  local 
circuit  H  R  b.  The  relay  in  its  turn  operates 
the  sounder  M  through  a  second  local  circuit, 
including  the  voltaic  cell  v.  It  is  necessary  to 
make  sure  that  the  delicate  receiving  apparatus 
is  completely  disconnected  from  the  antenna 
while  the  latter  is  being  used  for  sending.  Some- 
times an  automatic  switch  is  used  which  will 
not  permit  the  induction  coil  to  be  excited  unless 
the  receiver  is  cut  off.  In  some  installations  the 
coherer  and  its  immediate  connections  are  shut 
up  in  a  metal  box  to  keep  accidental  waves  from 
impinging  upon  the  coherer. 

The  sending  key  used  in  wireless  telegraphy 
differs  only  in  mechanical  details  from  the  ordi- 
nary Morse  key  of  wire  telegraphy.  An  ordinary 
Morse  key  is  illustrated  in  Fig.  53,  and  a  form 
of  wireless  telegraph  key  in  Fig.  54.  The  wire- 
less key  has  to  send  a  much  stronger  current  to 
the  induction  coil  than  that  which  an  ordinary 
wire  -  telegraph  key  controls,  so  its  contact  is 
larger  and  stouter.  Moreover,  there  is  a  pos- 


152  WIRELESS  TELEGRAPHY 

sibility  of  the  operator  receiving  a  severe  electric 
shock  from  the  induction  coil;  so  the  insulating 
handle  is  made  more  massive.  There  is  apt  to 
be  some  sparking  at  the  key  contact  on  breaking 


FIG.  53. — Ordinary  Wire-      FIG.  54. — Form  of  Wireless 
Telegraph  Morse  Key.  Telegraph  Morse  Key. 

circuit,  and  certain  forms  of  key  are  designed  to 
overcome  this,  in  some  cases  by  breaking  the 
contact  under  oil,  and  in  others  by  breaking  the 
contact  between  the  poles  of  an  electromagnet. 

Morse  Alphabets  or  Codes 

In  sending  signals,  the  contacts  of  the  key  are 
made  by  the  operator  in  conformity  with  the 
Morse  code.  There  are  unfortunately  two  tele- 
graph codes — the  American  Morse  code,  or  that 
in  almost  universal  use  on  the  North  American 
continent,  and  the  international  Morse  code,  or 
continental  Morse  code,  in  use  in  practically  all 
other  parts  of  the  world.  The  American  Morse 
code  was  introduced  in  the  early  days  of  the 
Morse  system  in  the  United  States.  In  Europe, 


WIRELESS  TELEGRAPH  WORKING      153 

about  the  same  time,  a  number  of  different  al- 
phabets or  codes  sprang  into  existence,  in  differ- 
ent places.  The  frequent  transition  and  inter- 
communication of  messages  among  European 
countries  was  soon  hampered  by  differences  in 
Morse  alphabet,  so  that,  by  international  con- 
vention, the  present  Continental  code  was  ar- 
rived at  and  adopted.  Unfortunately,  the  date 
of  this  convention  was  prior  to  the  introduction 
of  Atlantic  cables  and  fast  ocean  steamers  so 
that  America  was  not  a  party  to  the  conference. 
The  two  codes  are  presented  in  Fig.  55,  so  far 
as  concerns  the  use  of  the  English  language.  It 
will  be  seen  that  the  signals  for  a,  b,  d,  e,  g,  h,  i, 
k,  m,  n,  s,  t,  u,  v,  w,  and  4  are  common  to  both 
codes;  while  the  signals  for  c,  f,  j,  1,  o?  p,  q,  r,  x, 
y,  z,  i,  2,  3,  5,  6,  7,  8,  9,  o, . ,  ?,  and  !  are  different. 
In  both  codes  the  dot  is  the  standard  element  of 
length.  A  dash  has  the  length  of  three  dots, 
and  the  space  separating  dots  or  dashes  in  a 
letter  are  of  dot  length;  except  in  the  American 
letters  c,  o,  and  z  which  are  called  spaced  letters, 
and  in  which  there  is  an  extra  space  of  two  dots' 
length.  The  American  1  is  a  six-dot  length  dash, 
and  the  Americai)  zero  is  a  nine-dot  length  dash. 
The  space  separating  adjacent  letters  is  three 
dots  long  and  the  space  separating  words,  six 
dots  long. 


i54  WIRELESS  TELEGRAPHY 

The  American  code  is  shorter  on  the  average, 
in  the  signaling  of  ordinary  English,  by  about 
5  per  cent.;  that  is  to  say  95  dot  elements  of 
American  code  will  be  equivalent  in  the  forma- 
tion of  letters  to  100  elements  of  international 
code,  so  that  the  American  code  is  swifter  by 
about  this  amount.  On  the  other  hand,  the 
spaced  American  letters  are  a  source  of  possible 
errors,  if  the  signaling  is  not  in  the  best  condi- 
tion, and  some  operators  who  are  practically 
acquainted  with  both  codes  maintain  that,  owing 
to  the  care  needed  in  sending  spaced  letters, 
there  is  no  sensible  difference  in  swiftness  be- 
tween the  codes.  It  is  certain  that  a  person  con- 
versant solely  with  one  of  these  codes  is  quite 
unable  to  read  messages  sent  in  the  other.  Listen- 
ing to  the  other  code  in  such  a  case  is  like  listening 
to  foreign  unknown  speech.  It  is  greatly  to  be 
regretted  that  this  confusion  of  telegraphic  lan- 
guage exists,  and  wireless  telegraphy  has  tended 
to  make  the  confusion  more  evident.  Most  of 
the  ships  carrying  wireless  telegraph  outfits  talk 
international  Morse.  Only  those  like  the  Fall 
River  Liners,  on  the  American  coast,  talk  Amer- 
ican. Some  ships  can  talk  in  either  code. 

Each  dot  contact  of  the  sending  key  must  be 
accompanied  by  at  least  one  discharge  of  the 
induction  coil  and,  therefore,  by  at  least  one 


WIRELESS  TELEGRAPH   WORKING 


155 


Cmttuintil 

American. 

fettiVunhl 

JU**«». 

A 

- 

w 

— 

— 

B 

- 

... 

X 



— 

C 



... 

y 



— 

D 

. 

- 

z 

— 

E 

- 

i 





F 



.-. 

2 





G 

— 

3 





H 

— 

- 

4 

6" 

-- 



J 





6 

. 

K 

— 

7 





L 

—  .. 

—  - 

8 





M 

; 

- 

9 





O 



»    • 

• 

P 





J 

. 

R 

i_. 

«*. 

sn 

..._- 

6 

- 

• 

t 

— 

T 

- 

; 



U 

-- 

- 

I 



V 

•• 

"" 

& 

FIG.  55. — Continental  and  American  Morse  Telegraph 
Codes. 


156  WIRELESS  TELEGRAPHY 

spark,  or  train  of  oscillating  spark  discharges  at 
the  spark-gap  &,  Fig.  52.  The  dashes,  on  the 
other  hand,  need  to  set  up  a  series  of  discharges. 
A  dot  may  thus  be  accompanied  by  a  single  inter- 
ruption at  the  vibrator  and  a  dash  by  three  or 
more  such  interruptions. 

Alternations  0}  Sending  and  Receiving. 

During  the  time  that  the  message  is  being  sent, 
the  operator  is  unable  in  the  ordinary  wireless 
system  to  receive  a  message,  or  to  know  whether 
the  receiving  operator  has  been  able  to  take  the 
signals.  In  ordinary  wire  telegraphy,  as  prac- 
ticed in  the  United  States,  the  receiving  operator 
can  "break"  the  line  circuit,  and  thus  notify  the 
sender  that  the  message  is  not  being  taken.  But 
the  receiver  of  a  wireless  telegraph  message 
cannot  ordinarily  stop  the  sender,  and  the  sender 
goes  on  until  either  the  message  has  been  con- 
cluded, or  until  he  deems  it  prudent  to  turn  his 
switch  and  let  the  receiver  send  back  encourage- 
ment to  proceed.  For  the  same  reasons,  it  is  not 
uncommon  for  the  sender  to  repeat  a  message, 
as  soon  as  he  has  finished  it,  in  order  that  the 
distant  receiving  operator  may  check  what  he 
has  written  against  the  repetition,  so  as  to  avoid 
mistakes.  Attempts  have  already  been  made  to 
send  and  receive  messages  simultaneously  by  the 


WIRELESS  TELEGRAPH  WORKING      157 

same  antenna,  but  such  operations  can  only  be 
regarded  at  present  as  in  a  subsidiary  stage.  As 
soon  as  the  difficulties  in  the  path  of  ordinary, 
simple,  simplex  to-and-fro  wireless  telegraph 
signaling  have  been  overcome,  it  will  be  time  to 
attend  to  the  development  of  more  intricate 
methods. 

The  Insulation  of  Antennas 

The  insulator  or  insulators  which  support  the 
sending  antenna  have  to  be  maintained  in  good 
order,  because  they  must  withstand  high  voltage 
without  appreciably  leaking  or  sparking.  Where- 
ever  the  antenna  wire  or  wires  come  in  contact 
with  any  substance,  an  insulator  has  to  be  used, 
especially  where  there  is  a  chance  of  moisture 
from  rain,  dew  or  fog.  The  antenna  may  itself 
be  an  insulated  or  rubber-covered  wire.  This  is, 
however,  of  but  little  service  except  in  preventing 
atmospheric  discharges  from  driving  wind  and 
snow.  On  the  other  hand,  the  insulation  of  an 
antenna  used  solely  for  receiving  messages  need 
not  be  so  carefully  maintained.  In  order  to  send 
messages,  powerful  voltages  must  be  used  and 
insulated;  but  in  order  merely  to  receive  mes- 
sages, or  to  listen  to  what  is  going  on  in  the 
neighboring  ether  so  far  as  concerns  long  electro- 
magnetic waves,  only  feeble  voltages  are  pro- 


158  WIRELESS   TELEGRAPHY 

duced ;  so  that  while  insulation  is  proper,  it  need 
not  be  safeguarded  elaborately.  A  bare  copper 
wire  fastened  around  a  branch  oi  a  tree  and 
touching  the  boughs  or  trunk  at  several  places 
on  the  way  down,  may  often  enable  messages  to 
be  received.  Such  a  wire  could  hardly  be  used 
for  sending  messages  to  a  distance. 

Heights  of  Antennas 

Antennas  are  carried  to  various  heights,  as 
already  mentioned.  Since  the  trouble  and  ex- 
pense of  construction  increase  rapidly  above  a 
heigh;,  of  30  meters  (32.8  yards),  very  high  masts 
are  only  installed  for  very  long  distance  trans- 
mission. The  greatest  height  to  which  they 
have  been  thus  far  carried  is  128  meters  (420 
feet),  in  the  form  of  a  steel  chimney.  On  board 
ship  the  mast  height  usually  limits  the  elevation 
of  the  antenna  to  about  30  meters  (98.4  feet).  A 
widely  extending  antenna  is  not  so  useful  for 
receiving  as  for  sending,  except  when  the  waves 
are  much  longer  than  four  times  the  mast  height ; 
because  although  side  expansion  no  doubt  affords 
an  increase  of  catchment  area  for  passing  waves, 
there  is  little  doubt  that  the  area  from  which 
energy  is  absorbed  by  a  receiving  antenna  is 
fairly  wide  (see  page  84),  even  when  the  antenna 
consists  of  but  a  single  vertical  wire.  On  the 


WIRELESS  TELEGRAPH  WORKING      l>9 

other  hand,  increasing  the  height  of  a  receiving 
antenna  increases  the  energy  that  can  be  scooped 
out  of  a  passing  wave-train  approximately  as  the 
square  of  the  elevation,  provided  that  resonance 
is  maintained;  so  that  increase  of  height  always 
aids  long-distance  reception. 

The  ordinary  height  of  a  shore  antenna  mast 
is  about  45  meters  or  150  feet.  If  a  mast  is  ob- 
served on  the  seashore  with  a  little  wooden  house 
at  the  base,  it  is  highly  probable  that  it  is  con- 
structed for  wireless  telegraph  purposes.  If  a 
group  of  several  wires  can  be  seen  to  festoon 
from  the  masthead  to  the  house,  the  existence, 
either  in  the  present  or  past,  of  a  wireless  tele- 
graph station  may  safely  be  assumed. 

Although  the  usual  method  of  installing  an 
antenna  is  to  have  the  same  insulated  at  the  top 
and  well  grounded  at  the  base,  yet  other  methods 
are  possible  and  are  occasionally  employed.  For 
example,  the  antenna  may  consist  of  an  arch  or 
vertical  loop,  either  one  or  both  ends  of  which 
may  be  grounded.  Again,  the  ground  connec- 
tion of  an  antenna  may  be  dispensed  with  en- 
tirely, and  an  insulated  metallic  plate  used  in  its 
place,  the  plate  being  preferably  supported 
parallel  to  the  ground  and  at  a  height  of  about 
2  meters  (6.5  feet)  above  it.  Such  a  plate  is 
equivalent  to  an  air  condenser  inserted  in  the 


160  WIRELESS  TELEGRAPHY 

antenna  path  near  the  ground  connection.  Or, 
the  ground  may  be  dispensed  with,  and  a  hori- 
zontal wire  may  be  run  at  a  short  distance  above 
the  ground.  A  number  of  such  variations  of 
installation  have  been  suggested  or  used  at  differ- 
ent times;  but  while  they  introduce  differences 
of  action  in  detail,  they  usually  conform  to  the 
same  broad,  fundamental  principles  of  action  as 
the  ordinary  lightning-rod  type  of  vertical  wire 
antenna  with  grounded  base. 

Power  Required  for  a  Wireless  Telegraph 
Sending  Station 

In  order  to  receive  wireless  signals,  no  power 
has  to  be  expended  beyond  the  almost  infinitesi- 
mal amount  supplied  by  voltaic  cells  in  the  local 
circuit  or  circuits  of  the  receiver.  But  in  order 
to  transmit  signals  to  a  distance,  very  appreciable 
power  must  be  supplied.  For  distances  under 
100  kilometers  (60  miles),  the  power  supplied 
does  not  usually  exceed  3  kilowatts  (4  horse- 
power), and  is  sometimes  considerably  less.  As 
the  distance  increases,  the  power  absorbed  is 
likewise  increased.  A  few  stations  use  50  kilo- 
watts (67  horse-power)  or  more.  As  we  have 
already  seen,  the  antenna  radiates  the  power  by 
jerks,  or  with  long  intermissions;  so  that  al- 
though in  sending  with  the  key  depressed,  the 


WIRELESS  TELEGRAPH  WORKING       161 

power  supplied  to  the  primary  winding  of  the 
induction  coil  may  be;  say,  2  kilowatts  (2§  horse- 
power), the  antenna  may  be  varying  in  power 
between  200  kilowatts  and  o. 

In  the  neighborhood  of  a  city,  or  of  an  electric 
lighting  system,  the  power  required  may  be 
taken  from  electric-light  mains.  The  same 
facility  may  be  afforded  on  vessels  electrically 
lighted.  Otherwise,  a  small  gas,  oil  or  steam- 
engine  has  to  be  installed,  in  order  to  generate 
the  power  required. 

Contrast  Between  Power  Required  jor  Wire  and 
Wireless  Telegraphy 

The  contrast  between  wire  and  wireless  teleg- 
raphy is  nowhere  more  marked  than  in  respect 
to  power.  In  wire  telegraphy,  over  a  distance 
of,  say,  200  kilometers  (120  miles),  the  power  re- 
quired for  the  transmission  of  messages  is  per- 
haps 8  watts  at  the  generator  (5.9  foot-pounds 
per  second,  or  0.0107  horse-power).  In  wireless 
telegraphy,  working  over  the  same  distance,  8 
kilowatts  (10.8  horse-power)  may  be  expended, 
or  a  thousand  times  more.  Even  then,  the  wire- 
less system  is  only  rendered  practicable  by  the 
enormously  greater  sensitiveness  of  the  receiving 
apparatus  it  employs.  In  the  one  case,  the 
electromagnetic  waves  are  guided  in  a  single 


1 62  WIRELESS  TELEGRAPHY 

beam  to  their  destination  by  a  wire,  and  the  only 
serious  loss  is  by  leakage  over  the  insulators  along 
the  line,  or  by  imperfect  conduction  and  internal 
warming  of  the  line  wire.  In  the  other  case,  the 
power  is  scattered  in  every  direction  and  only  an 
extremely  small  fraction  can  be  picked  up  and 
utilized  at  the  distant  receiving  station.  Never- 
theless, the  fact  that  a  wire  can  be  dispensed  with 
is  so  wonderful,  that  we  may  be  glad  to  obtain 
wireless  telegraph  communication  at  any  rea- 
sonable cost,  and  not  be  over  anxious  to  cavil  at 
the  waste  of  energy. 

Spreading  0}  Waves  jrom  the  Sending  Station 

We  have  already  seen  that  electromagnetic 
waves  employed  in  wireless  telegraphy  start  off 
as  hemispherical  waves,  expanding  at  light- speed 
in  all  directions.  The  hemispherical  formation 
fairly  commences  about  a  half-wave  length  from 
the  sending  antenna.  Inside  that  radius  the 
wave  is  much  more  complex  in  form.  Theoreti- 
cally, the  expansion  goes  on  indefinitely,  allowing 
for  the  curvature  of  the  earth.  Practically,  very 
little  has  yet  been  determined  about  the  matter, 
except  that  signals  from  a  sending  antenna  have 
been  detected  in  balloons  at  moderate  elevations 
above  the  earth  not  far  from  the  antenna,  and 
also  by  observers  on  the  earth  at  a  distance  not 


WIRELESS  TELEGRAPH   WORKING      163 

exceeding  5,000  kilometers  (3,000  miles)  in  any 
direction.  No  receptions  of  messages  have  been 
yet  reported  over  a  greater  distance  than  this. 


FIG.  56. — The  Boston  Hemisphere  of  the  Globe  in 
Stereographic  Projection;  or  the  Hemisphere  with 
Boston  as  Pole. 

Fig.  56  represents  the  spreading  of  the 
waves  from  the  sending  station  according  to  the 
existing  theory.  The  sending  station  is  sup- 
posed to  be  located  at  Boston,  Mass.,  which  is, 


164  WIRELESS  TELEGRAPHY 

therefore,  placed  at  the  pole  of  the  earth,  for  the 
purpose  of  this  discussion.  Each  of  the  dotted 
circles  i,  2,  3,  .  .  .  9  represents  an  over- 
sea distance  of  ten  degrees,  or  600  nautical  miles 
(i,m  kilometers  or  690  statute  miles).  In  pass- 
ing from  one  circle  to  the  next,  the  waves  will 
take  -2To~th  of  a  second,  or  3,700  microseconds. 
The  complete  journey  from  Boston  at  the  pole, 
to  the  equator  on  circle  9,  or  one-quarter  round 
the  world,  would  occupy  -^j-th  second.  It  will 
be  observed  that  the  outgoing  waves  strike  New- 
foundland, Bermuda  and  the  Great  Lakes,  about 
the  same  instant,  -^-yc"^  °f  a  second  after  being 
emitted.  Cuba  and  the  Greenland  coast  would 
be  struck  almost  simultaneously  in  -g-f-g-ths  sec- 
ond. When  the  waves  reach  England,  in  about 
S-fg-ths  second,  they  will  also  be  striking  Alaska, 
the  Pacific  coast,  Norway,  France,  Spain,  Mo- 
rocco and  the  Brazilian  shore.  They  will  also 
nearly  have  reached  the  North  Pole.  After 
•j-fg-ths  second,  Japan,  Thibet,  Persia,  Egypt,  the 
Gold  Coast,  Argentina  and  Peru  will  be  reached 
almost  simultaneously.  The  corresponding  de- 
velopment of  the  waves,  in  section  through  the 
center  of  the  globe  and  the  sending  station,  is 
presented  in  Fig.  57.  Boston  would  occupy  the 
position  P,  and  a  point  on  the  ocean  south  of 
Perth,  Australia,  would  occupy  the  antipodes  P'. 


WIRELESS  TELEGRAPH  WORKING      165 

The  expanding  waves  would  occupy  the  succes- 
sive shells  in,  222,  333,  and  444,  the  last  being 
attained  in  ^th  of  a  second.  The  equatorial 
circle  Q  222  Q  corresponds  to  the  circle  9  in 


FIG.   57. — Hypothetical   Expansion  of  Wireless  Tele- 
graph Waves  Over  the  Globe. 

Figure  56.  If  will  be  observed  that  after  the 
wave  passes  the  equatorial  circle  Q  Q,  it  narrows 
its  circle,  and  when  it  reaches  the  antipodes  P', 
it  has  gathered  all  its  feet  to  this  point.  This 
should  mean  that  the  weakening  of  the  waves, 
which  occurs  by  expansion  at  the  outset,  dimin- 
ishes slightly  on  the  surface  of  the  globe  after 


1 66  WIRELESS  TELEGRAPHY 

passing  the  equator,  and  the  signals  at  the  an- 
tipodes P'  should  be  relatively  stronger  than  at 
other  points  in  that  region.  All  this  is,  however, 
as  yet  entirely  inferential,  because  the  waves 
have  not  yet  been  detected  beyond  the  first  posi- 
tion 1,1,  about  one-eighth  of  the  distance  around 
the  globe,  or  half  way  to  the  equator. 

Moreover,  it  is  uncertain  as  to  what  happens 
to  the  expanding  waves  in  the  upper  regions  of 
the  earth's  atmosphere,  where  no  balloon  can 
ascend  to  take  observations.  Air  is  an  excellent 
insulator  at  ordinary  pressures;  but  when  air  is 
exhausted  from  a  vacuum-tube,  the  dregs  of  air 
remaining  within  can  be  made  to  conduct  electric 
discharges  better  than  sea- water.  The  pressure 
of  the  air  is  a  maximum  at  the  earth's  surface, 
and  dwindles  indefinitely  as  the  earth's  surface 
is  departed  from.  At  an  elevation  of  from  20 
to  100  kilometers  above  the  earth  (12  to  60  miles), 
depending  upon  local  conditions,  there  must  be 
strata  of  rarified  air  having  as  low  a  density  as 
that  produced  in  such  vacuum-tubes.  It  is  un- 
certain whether  very  feeble  electric  forces  can 
evoke  conduction  in  such  rarified  air.  That  is 
to  say,  it  is  quite  possible  that  conduction  may 
occur  in  laboratory  vacuum-tubes  after  the  rari- 
fied air  has  been  modified,  or  ionised  as  it  is 
called — by  the  action  of  a  relative  powerful  elec- 


WIRELESS  TELEGRAPH  WORKING      167 

trie  flux,  and  that  in  the  absence  of  such  degrees 
of  electric  flux-density,  the  rarified  air  would  fail 
to  conduct.  If  there  is  a  shell  of  rarified  air 
above  the  earth  at  a  height  of,  say,  70  kilometers 
(roughly  40  miles)  which  suddenly  conducts  like 
sea-water,  then  the  electric  flux  would  cease  to 
expand  into  the  space  beyond,  but  would  skim 
along  the  interior  wall  of  this  conducting  shell. 
The  waves  would  then  be  confined  to  expansion 
sideways  between  the  earth's  conducting  surface 
below  and  the  internal  wall  of  the  concentric 
globe  of  rarified  air  above.  This  would  tend  to 
reduce  the  attenuation  or  weakening  of  the  waves 
markedly;  because  beyond  the  radius  of  70  kilo- 
meters, the  expansion  would  continue  in  but  two 
dimensions — longways  and  sideways — instead  of 
three  dimensions — including  height.  On  the 
other  hand,  however,  there  might  be  layers  of 
rarified  air  which  conducted  even  under  very 
feeble  electric  flux,  and  yet  the  conduction  might 
be  gradual  instead  of  sudden.  If  the  conduction 
increased  gradually  to  a  maximum  through  many 
kil  )meters  of  ascent,  there  would  be  loss  of  energy 
by  reason  of  imperfect  conduction  and  eddy- 
currents  in  the  transitional  layers,  so  that  the 
benefit  due  to  ultimate  confinement  of  the  waves 
within  the  rarified  shells,  might  be  more  than  lost 
by  waste  of  energy  in  this  partial  conduction. 


1 68  WIRELESS  TELEGRAPHY 

The  whole  subject  of  wave  contour  at  great  dis- 
tances must  remain  in  abeyance  until  sufficient 
measurements  have  been  made  of  the  wave- 
strengths  at  different  distances  to  enable  the  con- 
tours to  be  inferred.  Very  little  information  is 
yet  obtainable  as  to  relative  wave-strengths  at 
great  distances,  partly  owing  to  the  difficulty  of 
measuring  extremely  feeble  wave-intensities  and 
partly  owing  to  atmospheric  variations,  to  be 
referred  to  later.  At  short  distances,  i.e.,  100 
kilometers  (about  60  miles)  or  less,  the  few  re- 
sults obtained  appear  to  indicate  that  the  energy 
received  diminishes  roughly  as  the  inverse  square 
of  the  distance,  or  in  conformity  with  simple 
hemispherical  expansion. 

Experimental  Apparatus  for  a  Range  0}  a  Few 
Meters 

For  simple  experimental  and  demonstrative 
purposes,  apparatus  is  readily  obtainable  that 
will  produce  recognizable  signals  at  very  short 
range  and  with  an  insignificant  expenditure  of 
power.  The  apparatus  of  Fig.  58  consists  of  a 
small  induction  coil  with  vibrator,  to  which  the 
primary  current  from  an  external  voltaic  battery 
is  admitted  by  the  Morse  key  K.  The  terminals 
T  t  are  for  connection  to  a  voltaic  battery.  The 
secondary  winding  of  the  induction-coil  charges 


WIRELESS  TELEGRAPH  WORKING      169 

the  insulated  double-rod  system  R  R.     By  this 
means  short  waves  are  thrown  off  which  strike 


FIG.  58. — Simple  Form  of  Wireless  Sending  Apparatus 
for  Transmitting  to  a  Distance  of  a  Few  Metres. 


FIG.  59. — Receiver  for  Experimental  Wireless  Telegraph 
Set  of  Few  Metres  Range. 

the  receiver  in  the  vicinity.  A  form  of  receiving 
apparatus  capable  of  being  used  with  such  a  set 
is  indicated  in  Fig.  59.  Here  the  horizontal 


170  WIRELESS  TELEGRAPHY 

glass-tube  coherer  C  is  connected  to  the  binding- 
posts  3,  3  on  the  wooden  base.  Short  projecting 
wires  may  be  clamped  in  these  to  help  seize  the 
passing  waves.  A  voltaic  cell  is  also  connected 
through  binding  posts  2,  2  with  the  coherer, 
through  the  relay  R.  This  relay  is  actuated 
when  the  coherer  responds,  and  closes  a  local 
circuit  through  the  vibrating  electric  bell  B  and 


FIG,  60. — Simple  Neutral  Relay. 

another  voltaic  cell  connected  to  binding-posts 
i,  i.  The  ringing  of  the  bell  not  only  gives  the 
signal,  but  also  agitates  the  coherer  and  restores 
its  normal  insulation,  after  the  passage  of  the 
wave. 

The  relay  R  is  represented  in  greater  detail  in 
Fig.  60.  A  pair  of  electromagnetic  coils  M  are 
wound  with  silk-covered  fine  copper  wire  con- 
nected at  the  ends  to  the  main  terminals  T  T, 
The  armature  of  the  relay  is  free  to  move  about  a 


WIRELESS  TELEGRAPH  WORKING      171 

horizontal  axis  through  a  small  play,  set  by  the 

two  uppermost  opposing  screws.     A  spiral  spring 

adjusted  in  tension  by  the  set-screw  S,  draws  the 

armature  away  from  the 

electromagnetic   poles, 

when  these  are  unexcited 

by    electric    current    in 

the  coils.    The  armature 

lever  then  rests  against 

an  insulating  stop.    On 

being   attracted    by  the 

magnet  poles,  the  lever  _ 

FIG.  6 1,— Polarised  Relay, 
strikes  the  contact  point 

C,  thus  completing  a  local  circuit  through  the 
terminals  L  L,  and  wires  to  the  same  underneath 
the  base. 

In  long-distance  wireless  telegraphy  with  the 
coherer,  a  more  delicate  form  of  relay  is  gener- 
ally employed,  such  as  that  shown  in  Fig.  61. 
In  this  form  the  delicate  lever  plays,  under  glass 
cover,  about  a  vertical  axis  and  the  adjustment 
is  provided  by  the  screw  on  the  side. 

The  simple  apparatus  of  Figs.  58  and  59  for 
sending  messages  across  a  hall,  or  from  one 
building  to  another  near  by?  is  of  some  practical 
importance,  because  a  wireless  telegraph  station 
equipped  to  receive  messages  over  a  distance  of 
hundreds  of  kilometers  or  miles  may  not  disdain 


172  WIRELESS  TELEGRAPHY 

to  employ  an  even  still  simpler  apparatus  of  the 
same  character,  for  testing  purposes.  An  ordi- 
nary vibrating  electric  bell,  such  as  that  shown 
at  B  in  Fig.  59,  when  excited  by  a  voltaic  cell, 
rapidly  makes  and  breaks  its  circuit  at  the  con- 
tact point  of  the  vibrator.  Each  such  interrup- 
tion is  usually  accompanied  by  a  little  spark  at 
the  vibrator  contact,  and  a  feeble  electric  wave, 
or  very  brief  wave-train  is  thrown  off  from  the 
circuit.  Such  an  apparatus  may  be  installed, 
say,  on  the  wall  of  the  wireless  telegraph  station, 
and  excited  by  pressing  an  ordinary  push  button 
at  the  receiving  operator's  desk.  The  operator 
wishing  to  ascertain  whether  his  receiving  appa- 
ratus is  in  order,  may  do  so  in  the  absence  of  any 
incoming  signals,  by  pressing  the  bell-button. 
The  feeble  electromagnetic  waves  thrown  off 
from  the  bell  wires  are  thus  enabled  to  attach 
themselves  to  the  antenna  wire  or  wires,  and  so 
to  produce  a  feeble  signal  that  the  operator  can 
recognize. 


CHAPTER   XII 

i 

TUNED   OR   SELECTIVE    SIGNALING    SYSTEMS 

The  Problem  0}  Selective  Signaling 

IN  the  last  chapter  we  considered  wireless  tele- 
graph working  between  two  stations  to  the  exclu- 
sion of  all  others  within  the  working  range.  But 
the  earth's  atmosphere  is  no  longer  an  Eden  with 
but  a  single  pair  of  occupants.  In  most  parts 
of  the  civilized  world,  the  actual  problem  is  how 
to  communicate  with  the  station  that  is  wanted, 
and  yet  to  keep  out  of  communication  with  dis- 
interested stations. 

Nature  of  Interference 

When  a  ship  carrying  an  ordinary  untuned 
coherer  receiving  set  occupies  a  position  at  which 
wave  signals  are  passing  from  only  one  sending 
station,  the  coherer  is  able  to  transmit  those 
signals  correctly  to  the  Morse  sounder,  or  ink- 
writer,  in  its  local  circuit.  The  same  may  be 
true  if  there  are  two  sending  stations  working 
simultaneously,  one  of  which  produces  much 


174  WIRELESS  TELEGRAPHY 

more  powerful  wave  signals,  at  the  ship's  posi- 
tion, than  the  other.  The  adjustment  of  the 
coherer  may  be  such  that  the  signals  which  are 
feebler,  either  owing  to  greater  distance,  or 
weaker  transmitting  apparatus,  are  unable  to 
interfere,  and  only  the  stronger  signals  are  re- 
corded on  board  the  ship.  If,  however,  there 
are  two  or  more  stations  in  the  neighborhood 
sending  signals  simultaneously,  and  the  inter- 
secting waves  from  these  stations  are  about 
equally  strong,  the  ship's  coherer  tends  to  re- 
spond to  all  of  the  signals,  or  to  give  an  unintel- 
ligible mixed  record.  It  is  true  that  if  the  waves 
from  the  different  competing  sending  stations 
have  different  lengths,  that  length  of  wave  which 
most  nearly  conforms  to  the  quadruple  of  the 
equivalent  ship's  antenna  height  will  preponder- 
ate in  strength.  Nevertheless,  in  an  untuned 
system,  the  other  waves  are  likely  to  interfere. 
This  is  partly  because,  as  we  have  already  seen, 
a  simple  sending  antenna,  not  tuned  in  connec- 
tion with  an  auxiliary  discharging  condenser, 
tends  to  emit  very  short  wave-trains,  that  are 
virtually  but  solitary  waves  with  tails  to  them; 
and  partly  because  a  simple  coherer,  at  the  base 
of  a  simple  receiving  antenna,  does  not  admit  of 
much  resonant  building  up  of  voltage,  even  with 
long  wave-trains. 


TUNED  OR  SELECTIVE  SIGNALING      175 

Auditory  Selection 

This  interference  and  jumbling  together  of 
signals  from  different  sending  stations  in  the 
neighborhood  was  soon  found  to  constitute  a 
menace  to  wireless  signalling  on  any  extensive 
scale,  especially  with  the  coherer  type  of  receiv- 
ing instrument.  With  other  types  of  receiver 
which  operate  through  a  telephone,  less  trouble 
from  interference  is  liable  to  be  felt.  This  is  for 
the  reason  that  in  a  telephone  receiver  the  signals 
usually  have  a  buzzing  sound,  or  possess  a  defi- 
nite semi-musical  tone.  The  pitch  of  the  tone 
corresponds  to  the  frequency  of  the  induction 
coil  vibrator  at  the  sending  station;  or,  as  it  is 
termed,  the  group- frequency;  i.e.,  the  number  of 
impulses  per  second  delivered  to  the  sending  in- 
duction coil  when  the  sender's  key  is  held  down 
(see  Fig.  33),  or  to  the  number  of  groups  of  waves 
emitted  per  second.  As  a  general  rule,  different 
sending  stations  do  not  employ  just  the  same 
group  frequency,  or  pitch  of  vibrator,  so  that  the 
characteristic  buzz  or  tone  of  the  signals  heard 
in  the  receiving  telephone  is  different  for  differ- 
ent stations.  When,  therefore,  a  number  of  sta- 
tions are  sending  messages  simultaneously  in  the 
neighborhood,  an  untuned  receiving  telephone 
set  will  render  them  all  audible  at  once.  If  they 


176  WIRELESS  TELEGRAPHY 

have  all  exactly  the  same  tone,  it  would  be  im- 
possible to  make  anything  of  the  jumble  of 
signals;  but  if,  as  usually  happens,  the  tones  are 
appreciably  different,  the  conditions  resemble  the 
jumble  of  sounds  maintained  in  a  reception  room, 
when  many  individuals  are  speaking  close  by  at 
once.  It  is  often  possible,  with  a  little  effort,  to 
focus  attention  on  one  particular  succession  of 
tones  and  mentally  to  read  the  signals  they  con- 
tain, to  the  exclusion  of  all  the  others. 

Need  for  Resonant  Selection 

There  is,  however,  a  limit  to  the  possibilities 
of  deciphering  one  tone  of  telephonic  signals  from 
among  a  crowd.  If  a  powerful  antenna  near  a 
civilized  seashore  is  connected  untuned  to  a 
sensitive  telephone  employing  receiver,  a  regular 
babel  of  signals  is  frequently  to  be  heard.  Some 
of  these  are  from  shore  stations  nearby,  others 
from  distant  shore  stations,  and  yet  others  from 
ships  at  sea.  As  we  look  upon  the  surface  of  a 
large  lake,  or  of  the  sea,  we  usually  discern  waves 
or  ripple  trains,  which  are  crossing,  superposing, 
or  intersecting  in  endless  variation.  A  calm, 
unruffled  water  surface  is  the  exception.  The 
same  conditions  now  apply  to  the  atmospheric 
ether  in  civilized  districts,  so  far  as  concerns  long 


TUNED  OR  SELECTIVE  SIGNALING      177 

electromagnetic  waves.     The  atmospheric  ocean 
is  rarely  quiescent. 

Adjustment  to  Resonance 

In  order  to  bring  about  sharply  selective 
signaling,  it  is  necessary  that  the  receiving  an- 
tenna and  apparatus  connected  therewith  should 
be  tuned  to  one  definite  wave-length,  so  as  to 
respond  to  waves  of  that  length  exclusively,  and 
also  that  the  sending  station  desiring  to  commu- 
nicate solely  with  that  receiver  should  be  tuned  to 
emit  as  long  wave-trains  as  possible,  possess- 
ing this  particular  wave-length.  Theoretically,  it 
should  be  possible  to  adjust  an  antenna  into 
resonance  to  a  given  wave-length  within  any  de- 
sired degree  of  precision,  so  that  if  the  waves 
received  were  one  per  cent,  too  short,  or  too  long, 
they  would  fail  to  actuate  the  receiver.  There 
is,  however,  a  limit  to  precision  in  practical  tuning 
for  various  reasons.  About  five  per  cent,  above 
or  below  is  usually  regarded  as  satisfactory. 
That  is,  a  receiving  antenna  and  apparatus  can 
be  arranged  to  respond  to  a  given  wave-length  of 
arriving  signals,  and  not  ordinarily  to  respond  to 
waves  five  per  cent,  shorter  or  longer.  This' 
means  that  the  receiver  would  not  ordinarily 
recognize,  or  report  in  the  telephone  or  recording 


iy8  WIRELESS  TELEGRAPHY 

apparatus,  any  passing  waves  outside  of  these 
limits. 

There  are  various  ways  of  connecting,  sending 
and  receiving  antenna  circuits  in  order  to  tune 
them.  One  such  way  is  indicated  in  Fig.  62  for 
the  sending  apparatus,  and  in  Fig.  63  for  the 

B 


FIG.  62. — Particular  Set  of  Tuned  Sending  Connections. 

receiving  apparatus.  By  means  of  a  suitable 
switch,  or  group  of  switches,  the  change  may  be 
made  from  one  set  to  the  other,  that  is  from  send- 
ing to  receiving,  with  one  and  the  same  antenna. 
Referring  to  Fig.  62,  A  B  is  the  antenna  or  wire 
leading  thereto,  C  an  adjustable  coil  for  virtually 


TUNED  OR  SELECTIVE  SIGNALING      179 

altering  the  equivalent  height  and  wave-length 
of  the  antenna.  A  high-frequency  induction  coil 
of  relatively  few  turns  without  any  iron  core,  is 
indicated  at  L,  the  secondary  winding  L3  being 
connected  to  the  antenna,  and  the  primary  Lx  to 
the  secondary  terminals  of  the  spark  coil  S, 


FIG.  63. — Particular  Set  of  Tuned  Receiving 
Connections. 


through  adjustable  condensers  c  c*.     The  key  K 
is  in  the  primary  circuit  of  the  spark  coil  S. 

The  circuit  s,  c,  Lx,  cf  is  adjusted  to  oscillate 
electrically  at  the  required  frequency  and  the 
antenna  path  B,  A,  C,  La  is  also  adjusted  to 


180  WIRELESS  TELEGRAPHY 

oscillate  at  the  same  frequency.  It  generally 
results  that  there  are  at  least  two  different  fre- 
quencies, and  not  merely  one  frequency  of  oscil- 
lation set  up  in  such  a  system;  but  one  of  the 
frequencies  is  taken  as  the  effective  or  working 
frequency,  and  the  others  are  regarded  as  in- 
effective or  merely  parasitical. 

Turning  now  to  the  receiving  connections  of 
Fig.  63,  A  B  is  the  antenna  wire  and  C  an  adjust- 
able coil  as  before.  L  is  a  high-frequency  induc- 
tion coil.  In  the  secondary  circuit  of  this  coil  is 
an  adjustable  small  coil  1,  an  adjustable  con- 
denser c,  and  the  receiver  r,  connected  with  the 
local  telephone,  or  recording  instrument.  The 
antenna  path  is  adjusted  to  oscillate  at  the  re- 
quired frequency,  and  the  secondary  circuit  L2, 
1,  c,  r  is  also  adjusted  to  oscillate  at  this  frequency 
Under  these  circumstances  the  whole  receiving 
system  is  tuned  to  resonance  with  the  single  re- 
quired frequency,  or  wave-length. 

Simultaneous  Sending  and  Receiving  with  Aid 
of  Differential  Resonance 

When  tuning  is  carefully  and  effectively  car- 
ried out,  it  may  enable  remarkable  results  to  be 
accomplished.  For  instance,  it  has  been  found 
possible  to  receive  messages  over  an  antenna  at, 
say,  the  foremast  of  a  steamer,  and  at  the  same 


TUNED  OR  SELECTIVE  SIGNALING      181 

time  to  send  messages  in  another  wave-length, 
from  an  antenna  at  the  mainmast,  to  a  different 
and  perhaps  very  distant  station.  It  is  evident 
that  this  result  would  not  ordinarily  be  possible 
with  simple  untuned  apparatus  at  both  antennas, 
nor  would  it  be  possible  with  tuned  apparatus, 
if  the  frequency  and  wave-length  of  both  the 
sending  mast  and  the  receiving  mast  were  the 
same. 

Increase  oj  Transmission  Range  by  Means  of 
Resonance 

The  advantages  of  tuning  are  found  not  only 
in  the  elimination  of  interference  from  extraneous 
signaling  stations,  but  also  in  increase  of  sensibil- 
ity and  effective  signaling  range.  The  tuning  of 
the  sending  station  connections  permits  of  in- 
creasing the  length  of  the  train  of  waves  follow- 
ing each  discharge  of  the  spark  coil.  The  sym- 
pathetic tuning  of  the  receiving  station  connec- 
tions permits  of  building  up  a  resonant  current 
strength  in  the  receiver,  due  to  the  successive 
additions  of  impulses  from  the  successive  waves 
in  the  train.  By  this  means,  signals  which 
would  be  too  faint  to  be  recognizable  if  they  de- 
pended upon  the  impulse  of  a  single  wave,  be- 
come recognizable  by  the  cumulative  impulses  of 
a  number  of  successive  waves.  There  is  good 


1 82  WIRELESS  TELEGRAPHY 

reason,  therefore,  for  expecting  that  when  a  suit- 
able high-frequency  dynamo  is  developed  for  the 
continuous  maintenance  of  power  in  the  sending 
circuit  and  antenna,  a  further  great  increase  in 
effective  range  of  transmission  will  be  rendered 
possible. 

Reduction  of  Atmospheric  Disturbance  by  Means 
oj  Resonance 

Another  advantage  derived  from  tuning  is  in 
the  direction  of  minimizing  the  influence  of 
atmospheric  discharges.  An  antenna  is  a  sort 
of  lightning  rod,  and  the  taller  and  more  exten- 
sive it  is,  the  better  atmospheric  discharger  it 
tends  to  become.  The  atmosphere  contains 
electrically  charged  particles  or  free  electric 
charges.  Their  presence  may  be  accounted  for 
in  several  ways  that  need  not  here  be  discussed. 
Consequently,  an  antenna  is  apt  to  receive  a 
perpetual  stream  of  little  electric  discharges  from 
the  layers  of  air  near  its  top,  to  the  ground  at  its 
base.  This  action  is  quite  distinct  from  the 
much  more  powerful  discharges  which  may  occur 
over  the  antenna  in  the  presence  of  a  thunder- 
storm in  the  vicinity.  During  such  a  thunder- 
storm it  is  usually  necessary  to  stop  signaling 
and  to  keep  the  antenna  grounded.  The  little 
atmospheric  discharges  become  objectionably 


TUNED  OR  SELECTIVE  SIGNALING      183 

noticeable  in  the  receiver,  and  sometimes  give 
rise  to  false  signals.  These  continuous  atmos- 
pheric disturbances  are  stated  to  be  more  notice- 
able and  troublesome  in  the  tropical  than  in  the 
temperate  zones,  but  they  vary  in  intensity  from 
day  to  day  and  hour  to  hour.  On  some  occa- 
sions they  are  almost  entirely  absent,  and  on 
other  occasions  they  are  markedly  prevalent. 
Distant  thunderstorms  and  atmospheric  dis- 
charges likewise  produce  noises  in  the  receiving 
telephone,  interfering  more  or  less  with  received 
signals.  Tuning  of  the  antenna  connections  is 
capable  of  reducing  atmospheric  disturbances, 
although  perhaps  not  of  eliminating  them  en- 
tirely. 

Multiple  Wireless  Telegraphy  by  Means  of 
Resonance 

The  results  of  tuning  have  even  been  carried 
further.  It  has  been  found  possible  to  send  two 
messages  simultaneously  over  one  and  the  same 
sending  antenna,  by  connecting  the  antenna  to 
two  different  coils,  and  auxiliary  oscillating  send- 
ing circuits,  or  to  two  different  sections  of  one 
and  the  same  coil.  Again,  it  has  been  found 
possible  to  receive  two  messages  simultaneously 
over  the  same  receiving  antenna  by  a  correspond- 
ing connection  to  two  oscillatory  receiving  cir- 


184  WIRELESS  TELEGRAPHY 

cults.  This  means  that  an  antenna  system  may 
be  arranged  to  emit  two  frequencies,  or  wave- 
lengths, simultaneously  and  independently.  In 
the  same  way,  one  antenna  may  be  arranged  to 
resonate  to  each  of  two  different  frequencies  or 
wave-lengths.  By  connecting  a  plurality  of  such 
oscillating  circuits  with  an  antenna,  it  is  theoreti- 
cally possible  either  to  send  or  to  receive  an  in- 
definite number  of  messages  simultaneously,  each 
in  a  definite  appropriate  wave-length.  Up  to  the 
present  time,  however,  but  little  use  has  been 
made  of  this  possibility.  A  multiple  system  of 
wireless  telegraphy  is  manifestly  more  compli- 
cated and  difficult  to  maintain  than  a  single  sys- 
tem. 

Limitations  of  Commumcability  Through 
Resonance 

Along  with  the  advantages  which  pertain  to  a 
tuned  or  selective  signaling  system,  there  is  one 
evident  disadvantage,  namely,  loss  of  communi  • 
cability.  It  is  all  very  well  for  a  ship  which  is 
tuned  to  receive  waves  say  300  meters  (328  yards) 
long,  or  at  a  frequency  of  one  million  cycles  per 
second,  to  be  able  to  carry  on  communication 
with  another  ship,  or  a  shore  station,  that  uses 
the  same  wave-length ;  but  a  third  station  having 
a  different  tuning  and  producing  waves,  say  400 


TUNED  OR  SELECTIVE  SIGNALING      185 

meters  long,  may  desire  to  communicate  with  the 
ship,  and  not  be  able  to  do  so,  either  from  not 
knowing  the  particular  wave  length  to  which  the 
ship  responds,  or  from  not  being  able  to  alter  his 
tuning  thereto.  It  is  partly  for  this  reason  that 
the  ordinary  wireless  equipment  on  board  ocean- 
going ves^ls  is  not  sharply  tuned.  It  is  desirable 
that  they  should  be  able  to  speak  to  all  comers 
within  normal  short  range. 

German  Practice 

Under  the  auspices  of  the  German  government 
a  dozen  stations  along  the  German  coast  are  all 
tuned  to  emit  waves  of  365  meters  (398.5  yards) 
in  length,  corresponding  approximately  to  a  fre- 
quency of  820,000  cycles  per  second.  These 
stations  have  a  signaling  range  of  about  200 
kilometers  (125  miles)  between  each  other,  or 
about  120  kilometers  (75  miles)  from  any  one 
shore  station  to  ships  in  its  neighborhood.  The 
ships  are  also  tuned  to  this  wave-length.  Under 
these  conditions  the  shore  stations  take  prece- 
dence. When  a  shore  station  calls,  ships  within 
range  are  instructed  not  to  speak  unless  called. 
Ships  are  also  instructed  not  to  call  a  shore  sta- 
tion needlessly,  or  beyond  the  normal  120  kilo- 
meter (75  miles)  range.  By  the  observance  of 
such  regulations  mutual  advantage  is  subserved, 


i86  WIRELESS  TELEGRAPHY 

as  well  as  the  keeping  of  ethereal  peace.  A  wire- 
less-telegraph etiquette  is  thus  gradually  becom- 
ing evolved. 

Difficulties  in  the  Way  of  Using  Reflectors  or 
Lenses  }or  Wireless  Light 

It  would  clearly  be  of  great  advantage  if,  in- 
stead of  radiating  waves  from  a  sending  station 
to  all  the  thirty-two  points  of  the  compass  at 
once,  there  were  some  convenient  means  of 
channeling  the  waves  into  a  beam,  capable  of 
being  sent  into  any  desired  direction.  This 
would  not  only  save  wasted  power,  but  would 
also  save  needlessly  stirring  up  the  ether  into 
noisy  signals  in  outlying  regions  where  other 
parties  are  trying  to  talk.  At  first  sight  it  would 
seem  that  because  we  can  readily  accomplish  this 
result  with  short-length  luminous  waves,  as  in 
the  search-light  beam  for  instance,  we  ought  like- 
wise to  be  able  to  accomplish  it  with  the  long 
waves  of  wireless  telegraphy.  The  trouble  is, 
however,  that  optics  leads  to  the  general  law  that 
both  reflectors  and  lenses  must  be  large  with 
respect  to  the  wave-length  of  the  waves  they  bend 
into  parallelism.  Thus,  with  half-micron  waves 
of  light,  a  tiny  mirror,  no  larger  in  diameter  than 
one  millimeter  (1-2 5th  inch),  would  cover  2,000 
wave-lengths. .  If  the  mirror  had  a  diameter  of 


TUNED  OR  SELECTIVE  SIGNALING      187 

less  than  half  a  micron  (-51$-$-$  inch)  or  less 
than  a  wave-length,  it  would  fail  to  serve  properly 
as  a  reflector.  In  the  same  way,  when  we  deal 
with,  say,  4oo-meter  (437  yards)  waves  of  invisible 
polarized  light,  it  would  take  a  metallic  surface  of 
more  than  400  meters  (437  yards)  square  to  make 
a  serviceable  reflector,  and  such  sizes  are  pro- 
hibitive. A  sending  station  situated  in  front  of 
a  very  steep  overshadowing  cliff,  of  fairly  con- 
ducting surface,  might  have  its  shore  side  shel- 
tered and  its  sea-going  waves  strengthened  by 
the  reflecting  surface  of  the  cliff;  but  such  locali- 
ties are  not  always  forthcoming;  nor  perhaps  is 
the  surface  soil  sufficiently  conducting  to  provide 
in  most  cases  a  satisfactory  reflector.  It  is  well 
known  that  a  ship  getting  behind  such  a  cliff  may 
be  sheltered  on  that  side  from  arriving  signals, 
or  in  other  words  that  such  cliffs  may  throw  long 
electromagnetic  shadows  beyond  them.  On  the 
other  hand,  a  vessel  lying  in  a  harbor  enclosed  by 
hills  which  do  not  rise  abruptly,  but  slope  to  the 
harbor,  will  often  receive  wireless  messages  from 
a  direction  over  the  hills,  the  waves  in  such  cases 
running  down  over  the  slope. 

Although  no  marked  degree  of  success  has 
hitherto  attended  the  erection  of  reflecting  ver- 
ticals, or  mirror  surfaces,  behind  sending  an- 
tennas, yet  care  has  to  be  taken  that  a  receiving 


1 88  WIRELESS  TELEGRAPH* 

antenna  is  suspended  reasonably  clear  of  high 
conductors  capable  of  casting  a  shadow,  Thus 
a  receiving  antenna  wire  suspended  behind  a 
steamer's  funnel  of  steel,  or  immediately  behind 
a  steel  mast,  would  be  likely  to  have  its  signals 
much  weakened,  if  not  entirely  absorbed.  For 
this  reason  an  antenna  is  usually  suspended  from 
a  rope  between  two  masts,  in  such  a  manner  as 
to  hang  free  from  both. 

Some  experimental  progress  toward  the  space 
direction  of  emitted  waves  has  been  recently  an- 
nounced by  the  addition  of  a  horizontal  an- 
tenna on  the  top  of,  and  proceeding  from,  a 
vertical  antenna.  This  construction  is  equiva- 
lent to  taking  say  an  8o-meter  (87.5  yards)  an- 
tenna, and  instead  of  setting  it  entirely  erect, 
placing  30  meters  (32.8  yards)  vertical  and  then 
carrying  the  remaining  50  meters  (54.7  yards) 
out  horizontally  in  a  straight  line  overhead.  In 
such  a  case  the  radiation  is  partly  polarized  in 
the  horizontal  plane,  and  partly  in  a  vertical  plane, 
the  resultant  being  a  sort  of  combination,  or 
elliptical  polarisation.  Such  waves  are  subject  to 
different  degrees  of  attenuation  in  different  direc- 
tions. It  is  found  that  the  radiated  waves  are 
strongest  in  the  direction  opposite  to  that  of  the 
horizontal  offset;  so  that  this  offset  in  the  an- 
tenna should  be  made  away  from  the  direction 


TUNED  OR  SELECTIVE  SIGNALING      189 

in  which  it  is  desired  to  send  the  waves.  In  the 
direction  of  the  offset,  the  waves  are  weak  and 
they  are  weakest  of  all  in  a  direction  nearly  at 
right  angles  to  the  offset. 

In  time,  it  is  to  be  hoped  and  expected  that  all 
large  vessels  will  carry  wireless  telegraph  appara- 
tus, and  that  all  lighthouse  stations  along  the 
ocean  shores  will  be  equipped  with  invisible  as 
well  as  visible  beams.  The  invisible  beams 
would  be  perceptible  by  electromagnetic  appara- 
tus far  out  at  sea,  and  also  in  foggy  or  hazy 
weather.  This  system  of  coast  protection  for 
arriving  vessels  is  already  commencing. 

Desirability  of  Means  for  Determining  Wave 
Directions  at  Sea 

When  a  wirelessly  equipped  steamer  ap- 
proaches the  coast  of  Europe  or  of  America,  she 
is  apt  to  be  apprised  of  the  proximity  of  land  in 
thick  weather  by  the  reception  of  signals  from 
coast  wireless  stations.  It  is  not  easy  to  deter- 
mine, however,  the  direction  whence  the  signals 
come,  or  the  bearing  of  the  station.  It  would  be 
advantageous  to  be  able  to  orient  the  received 
signals,  or  to  determine  their  direction  of  advance. 
By  suspending  two  or  more  antennas  far  apart, 
near  the  bow  and  stern  of  the  vessel  respectively, 
and  bringing  them  into  communication  through 


IQO  WIRELESS  TELEGRAPHY 

suitably  tuned  apparatus,  it  might  be  possible  by 
swinging  the  ship  to  find  the  approximate  bearing 
of  the  shore  station.  When  the  antennas  were 
in  line  with  the  station,  the  signals  would  reach 
a  maximum  (or  minimum),  and  when  the  line 
joining  them  was  at  right  angles  to  the  bearing 
of  the  station,  the  opposite  condition  should 
occur.  Such  a  procedure  is,  however,  objection- 
able, since  it  requires  the  ship  to  be  stopped  for 
observations,  or  at  least  turned  erratically  off  her 
course.  It  seems  likely  that  the  first  method  to 
be  found  available  will  be  for  each  shore  station 
to  erect  a  system  of  radiating  wires,  and  to  deter- 
mine the  direction  of  the  ship  by  tests  conducted 
upon  alternate  wires  or  pairs  of  these  wires.  The 
shore  might  then  inform  the  vessel  of  her  bearing, 
which  would  be  the  next  best  to  the  ship's  deter- 
mining the  shore  station  bearing  herself. 


CHAPTER   Xin 

MEASUREMENTS    OF   ELECTROMAGNETIC  WAVES 

Importance  of  Determining  Wave-lengths 

WE  have  already  seen  in  Chapter  VI  that  the 
length  of  the  waves  thrown  off  by  a  simple  verti- 
cal rod  oscillator,  discharging  into  a  perfectly 
conducting  level  ground,%  is  four  times  the  height 
of  the  rod;  so  that  in  the  case  of  a  simple  un- 
loaded antenna  of  the  type  represented  in  Fig. 
26,  we  can  form  a  close  estimate  of  the  wave- 
length emitted.  But  antennas  are  usually  loaded 
by  coils,  condensers,  or  other  apparatus,  so  that 
it  becomes  impossible  to  estimate  their  emitted 
wave-length  with  any  degree  of  reliability,:  More- 
over, the  wave-length,  which  is  not  of  great  im- 
portance in  simple  untuned  signaling,  becomes 
of  great  practical  importance  in  selective  signal- 
ing by  the  aid  of  resonance.  If  we  can  readily 
measure  the  length  of  the  waves  thrown  off  by  an 
antenna,  we  can  proceed  in  a  rational  and 
straightforward  way  to  tune  the  system,  and  also 
to  produce  any  required  wave-length  within 
reach. 

191 


IQ2  WIRELESS  TELEGRAPHY 

Method  jor  Determining  Wave-lengths. 

The  wave-length  of  an  oscillating  system  is 
measured  by  bringing  a  portable,  adjustable 
oscillation  system  into  electromagnetic  communi 
cation  with  it,  and  adjusting  the  portable  system 
until  resonance  is  observed  therein.  At  reso- 
nance, the  wave-length  of  the  tested  system  and  of 
the  portable  system  is  the  same.  The  wave- 
length of  the  portable  system  is  determined,  or 
computed,  from  its  adjustments,  and  the  wave- 
length of  the  tested  system  therefore  becomes 
known. 

The  simplest  kind  of  an  adjustable  oscillating 
system  consists  of  a  circuit  containing  a  coil  of 
wire  and  a  condenser.  Either  the  coil  may  be 
adjustable  in  its  length  and  virtual  number  of 
turns,  or  the  condenser  may  be  adjustable  in  the 
extent  of  its  effective  surface,  or  both  may  be 
adjustable  independently.  The  electromagnetic 
length  of  a  coil  is  conveniently  measured  and 
stated  in  centimeters  or  meters.  The  capacity 
of  a  condenser  is  also  conveniently  stated  in  the 
same  units  of  length.  When  the  oscillating  cir- 
cuit is  adjusted  to  resonance,  the  length  of  its 
wave  is  the  circumferential  length  of  that  circle 
whose  radius  is  the  geometric  mean  (square  root 
of  product)  of  the  coil  meters  and  condenser 


MEASUREMENTS  OF  WAVES  193 

meters.  Thus,  if  the  circuit  was  in  resonance 
when  the  coil  was  adjusted  to  1,000  meters,  and 
the  condenser  to  40  meters,  the  geometric  mean 
of  these  two  lengths  would  be  200  meters  and  a 
circle  with  this  radius  would  have  a  circumference 
of  1,257  meters  (1,375  yards)  which  is  the  wave- 
length of  the  resonant  circuit. 

Means  0}  Determining  Resonant  Condition  in 
Portable  Circuit 

In  order  to  recognize  when  the  portable  oscil- 
lation circuit  has  been  adjusted  to  resonance,  a 
galvanometer,  ammeter, 
or  current-indicator  suit- 
able for  high-frequency 
to-and-fro  currents,  or 
oscillations  is  placed  in 
the  circuit.  Fig.  64 
shows  one  form  of  such 
an  ammeter.  It  consists  FlG.  64.— Hot-Wire  Am- 
essentially  of  a  short  meter  or  Current  Meas- 
length  of  fine  platinum 

wire,  whose  ends  are  secured  to  fixed  supports. 
The  center  of  the  suspended  wire  is  fastened  to  a 
thread  that  runs  over  a  delicately  supported 
pulley,  carrying  a  pointer  or  index.  If  a  rapidly 
oscillating  electric  current  passes  over  the  fine 
platinum  wire,  the  wire  is  thereby  heated  and 


194  WIRELESS  TELEGRAPHY 

elongates.  The  elongation  is  detected  by  the 
sagging  of  the  thread  fastened  to  the  center,  and 
the  degree  of  elongation,  duly  magnified,  is 
caused  to  indicate  the  effective  strength  of  the 
current.  Care  has  to  be  taken  that  the  current 
passing  is  not  strong  enough  to  overheat  or  melt 
the  platinum  wire.  If  such  an  instrument  is 
inserted  in  a  resonant  oscillating  circuit,  the 
current  indicated  will  become  a  maximum  when 
resonance  is  produced. 

Thermo- galvanometer 

A  much  more  sensitive  galvanometer  or  cur- 
rent indicator  depending  also  upon  the  heat  pro- 
duced by  current  in  a  fine 
wire,  is  seen  in  Fig.  65. 
A  loop  of  fine  platinum 
wire  is  supported  close 
beneath,  but  not  touch- 
ing, the  soldered  junction 
of  two  wires  of  different 
metals,  forming  a  circuit 

FIG.   65  —Delicate  Ther-  which  is  delicately  sus- 
mo-Galvanometer.  .   . 

pended  in  a  permanent 

magnetic  field.  When  current  passes  through 
the  fine  platinum  wire,  some  of  the  heat  produced 
is  communicated  to  the  soldered  junction.  This 
raises  the  temperature  of  the  junction  and  pro- 


MEASUREMENTS  OF  WAVES  195 

duces  a  steady  thermo-electric  force  or  voltage,  as 
long  as  the  elevation  of  temperature  persists. 
The  voltage  sets  up  an  electric  current  in  the 
local  circuit  of  the  suspended  loop,  and  the  loop 
twists  or  tends  to  deflect  sideways  about  the  sus- 
pension, in  a  manner  and  to  a  degree  which  is 
greatly  magnified  by  an  attached  mirror  and  a 
beam  of  light  reflected  therefrom.  By  means  of 
this  thermo- galvanometer  very  feeble  oscillating 
currents  may  be  measured.  Some  of  the  best 
measurements  yet  published  concerning  the 
strength  of  signals  received  at  different  distances 
from  the  sending  station  have  been  obtained  by 
its  use. 

Disk  Galvanometer. 

Another  form  of  high-frequency  galvanometer 
is  indicated  in  Fig.  66.  It  consists  of  a  coil  of 
insulated  wire  having  comparatively  few  turns, 
and  supported  at  the  center  of  the  instrument, 
connected  in  the  circuit  under  test.  Inside  this 
fixed  coil  is  delicately  suspended  a  little  silver 
disk  s,  to  which  is  attached  a  small  mirror  m,  the 
two  being  carried  on  a  fine  quartz  fiber  about  30 
cms.  (12  inches)  long.  The  suspension  is  illus- 
trated separately  in  the  figure.  When  a  high- 
frequency  current  alternates  in  the  fixed  coil,  a 
feeble  alternating  current  of  the  same  frequency 


.96 


WIRELESS  TELEGRAPHY 


is  set  up  in  the  silver  disk,  and  the  electromagnetic 
attraction  between  the  current  in  the  coil  and  the 
current  in  the  disk  causes  the  disk  to  twist,  or 


FIG.  66. — Oscillating-Current  Galvanometer. 

deflect  about  the  axis  of  suspension.  The  deflec- 
tion is  magnified  and  rendered  measurable  by  a 
beam  of  light  reflected  from  the  mirror  m. 

The  portable  oscillation  circuit  is  brought  into 
electromagnetic  communication  with  the  tested 
circuit,  either  by  actual  contact  at  two  close 
points,  or  by  means  of  a  little  induction  coil  of 
very  few  turns. 


MEASUREMENTS  OF  WAVES  197 

Wave-measuring  Helix 

Another  type  of  portable  resonant  circuit  is 
made  in  the  form  of  a  helix,  or  close  spiral,  of  fine 
wire  wound  upon  an  insulating  rod  or  cylinder. 
Such  a  circuit  has  a  greatly  reduced  or  minified 
wave-length.  That  is,  a  wave  which  would 
occupy  a  length  of  say  50  meters  (54.7  yards)  of 
straight  wire,  would  perhaps  occupy  only  15 
centimeters  (6  inches)  of  this  wire  wound  in  a 
curl  or  helix.  When  the  antenna  to  be  tested  is 
in  excitation,  one  end  of  this  helix  is  presented 
near  to  but  not  touching  the  antenna,  and  the 
length  of  helix  which  will  set  up  resonance  is 
ascertained  by  running  a  grounded  conductor 
along  it.  Resonance  is  in  this  case  detected  by 
the  formation  of  a  glow  or  brush  discharge,  either 
from  the  end  of  the  helix  itself,  or  within  a  small 
vacuum- tube  placed  adjacent  thereto.  A  scale 
marked  along  the  rod  then  enables  the  observer 
to  read  off  the  wave-length  directly. 


CHAPTER    XIV 

INDUSTRIAL   WIRELESS   TELEGRAPHY 

The  Ocean,  the  Kingdom  of  Wireless  Telegraphy 

WIRELESS  telegraphy  has  already  come  into 
widely  extended  use  over  the  ocean.  It  has  not 
come  as  yet  into  extended  use  over  land.  The 
reasons  for  this  are  evident :  Wire  telegraphy  has 
already  held  undisputed  sway  overland.  Wherever 
there  has  been  developed  urgent  demand  for  the 
telegraph,  the  wire  has  been  run  to  meet  it.  But 
a  moving  ship  cannot  keep  up  wire  communica- 
tion with  the  land,  except  in  the  rare  instances 
where  a  ship  is  employed  in  laying  a  submarine 
cable.  Consequently,  wireless  telegraphy  has 
absolutely  undisputed  sway  over  the  surface  of 
the  ocean  in  reaching  ships  at  a  distance,  or  in 
reaching  ships  near  by,  when  visual  signals  can- 
not be  read,  as  at  night,  or  in  fog.  Already, 
wireless  telegraphy  has  done  splendid  work  in 
maintaining  communication  with  ships  at  sea. 
In  a  certain  sense  wireless  telegraphy  has  re- 
moved the  sea,  because  the  sense  of  isolation  in 
a  vessel  out  of  sight  of  land  is  almost  entirely  lost 
198 


INDUSTRIAL  WIRELESS  TELEGRAPHY     199 

when  messages  are  received  on  board  through  the 
ethereal  medium  of  the  air.  In  a  psychological 
sense,  distance  has  been  destroyed.  Moreover, 
since  the  sea  is  the  conducting  medium  or  broad 
conductor  for  guiding  wireless  telegrams,  we  may 
say  that  the  sea  has  ceased  to  divide  countries 
and  now  connects  them. 

Prospects  0}  Submarine  Cable  Telegraphy 

It  has  been  much  debated  whether  wireless 
telegraphy  would  render  submarine  telegraph 
cables  useless  and  cable  property  valueless.  Up 
to  the  present  time  it  has  not  done  so,  and  there 
is  no  immediate  prospect  of  its  doing  so.  Tak- 
ing, for  example,  the  islands  of  Great  Britain  and 
Ireland,  these  are  electrically  connected  with 
North  America  by  thirteen  submarine  cables, 
and  with  South  America  by  three  more.  Al- 
together there  are  about  sixty  wires  connecting 
Great  Britain  with  other  countries.  These 
cables  are  pouring  messages  into  the  islands  at 
an  average  rate  each  of,  say,  fifteen  words  per 
minute,  day  and  night  continuously.  It  would 
be  an  enormous  undertaking  to  replace  these 
cables  by  wireless  telegraphy,  without  any  refer- 
ence to  all  the  other  parts  of  the  world  served  by 
submarine  cables.  Up  to  this  date,  wireless 
telegraphy  has  probably  aided  submarine  cables 


200  WIRELESS  TELEGRAPHY 

by  bringing  telegraph  messages  to  and  from 
ships  at  sea,  for  transmission  by  cable,  more  than 
it  has  injured  cable  telegraphy  by  sending  mes- 
sages over  the  sea  that  would  otherwise  have 
gone  underneath.  On  the  other  hand,  however, 
it  must  be  remembered  that  wireless  telegraphy 
is  still  very  young,  and  that  it  has  already  made 
far  more  progress  in  its  brief  lifetime  than  did 
wire  telegraphy  in  the  corresponding  period  of 
its  life,  sixty  to  seventy  years  ago.  If  wireless 
telegraphy  continues  to  advance  in  the  future  at 
the  rate  it  has  maintained  in  the  past,  it  may  be 
that  at  some  distant  future  time  submarine  cables 
will  cease  to  be  laid,  and  their  work  surrendered 
to  their  wireless  rival. 

There  has  been  already  at  least  one  case  where 
wireless  telegraphy  has  supplanted  a  submarine 
cable,  and  that  is  between  the  U.  S.  army  stations 
of  Fort  St.  Michaels  and  Safety  Harbor,  Cape 
Nome,  across  Norton  Sound  on  the  coast  of 
Alaska,  a  distance  of  about  177  kilometers  (no 
miles).  The  cable  having  been  repeatedly 
broken  by  ice  on  this  ice-bound  coast,  the  tele- 
graph service  has  been  carried  on  for  about  two 
years  continuously  by  wireless  telegraphy. 

There  are  also  several  short  sea  distances  be- 
tween islands,  or  between  islands  and  mainland, 
which  have  recently  been  covered  by  wireless 


INDUSTRIAL  WIRELESS  TELEGRAPHY     201 

telegraph  equipments  instead  of  by  submarine 
cable.  One  of  these  is  between  Port  Blair  on  the 
Andaman  Islands  and  the  mainland,  Burma,  a 
distance  of  491.1  kilometers  (305.2  miles)  under 
the  auspices  of  the  Indian  government  tele- 
graph department,  the  traffic  averaging  about 
ninety  messages  per  month  each  way.  On  the 
other  hand,  a  number  of  important  long  sub- 
marine cables  have  been  laid  since  the  introduc- 
tion of  wireless  telegraphy. 

Peculiar  Difficulties  Incident  to  Wireless 
Telegraphy. 

One  of  the  difficulties  that  long-distance  wire- 
less telegraphy  has  had  to  deal  with,  which  will 
probably  always  have  to  be  expected,  is  disturb- 
ance from  thunderstorms  in  the  vicinity  of  a  sta- 
tion. Such  visitations  are  not  rare  during  sum- 
mer seasons,  and  usually  call  for  a  temporary 
suspension  of  traffic. 

A  curious  lesser  difficulty  that  long-distance 
wireless  telegraphy  has  to  meet  is  the  effect  of 
sunlight  upon  the  atmosphere.  Messages  can  be 
sent  and  signals  received  much  further  by  night 
than  by  day.  The  effect  of  sunlight  on  the  at- 
mosphere is  apparently  to  make  the  air  foggy  for 
the  long  waves  of  invisible  wireless  light.  The 
nature  of  the  action  has  not  yet  been  clearly 


202  WIRELESS  TELEGRAPHY 

demonstrated;  but  it  is  supposed  that  the  en- 
ergy of  sunlight,  or  short-wave  light,  in  im- 
pinging upon  the  ocean  of  air,  either  disrupts 
many  air-atoms  and  ionizes  them,  or  else  in- 
jects streams  of  ionized  matter  from  the  sun, 
thereby  leaving  floating  electric  charges  in  the 
air.  The  passage  of  electric  waves  through 
ionized  air  causes  work  to  be  done  in  displac- 
ing the  electrons,  or  electric  charges,  and  such 
energy  is  absorbed  from  the  stock  of  energy 
in  the  waves.  The  waves  therefore  become 
enfeebled,  or  absorbed,  in  a  manner  suggest- 
ing the  action  of  fog  upon  ordinary  light. 
The  degree  of  enfeeblement  during  daylight 
hours  is  not  uniform,  and  varies  from  day  to 
day  in  a  most  fluctuating  and  apparently  er- 
ratic manner.  This  means  that  in  order  to 
carry  on  wireless  telegraph  service  during  the 
worst  atmospheric  daytime  conditions,  there 
must  be  a  considerable  reserve  of  power  over 
and  above  that  necessary  during  the  night 
time  under  the  best  conditions. 

It  has  been  found  that  the  atmospheric  ab- 
sorption of  electromagnetic  wave  energy  oc- 
curs more  generally  and  more  powerfully  in 
the  tropical  zone  than  in  the  north  temperate 
zone,  presumably  on  account  of  the  greater 


INDUSTRIAL  WIRELESS  TELEGRAPHY     203 

relative  intensity  of  solar  radiation  upon  the 
atmosphere  in  the  tropics.  It  is  also  reported 
that  the  absorption  depends  in  a  marked  de- 
gree upon  the  length  of  the  electromagnetic 
waves  and  falls  off  very  rapidly  for  lengths 
of  wave  exceeding  3  kilometers  (1.86  miles) ; 
so  that  for  wave-lengths  of  3.75  kilometers 
(2.33  miles)  and  upwards,  corresponding  to 
frequencies  of  80,000  cycles  per  second  and 
under,  the  atmospheric  absorption  is  compara- 
tively small.  There  is  still  uncertainty  as  to 
the  nature  of  the  atmospheric  conditions  which 
produce  absorption;  but  the  great  and  sudden 
changes  in  the  strength  of  transatlantic  sig- 
nals, which  reveal  themselves  in  a  few  min- 
utes of  time,  suggest  the  presence  of  invisi- 
ble masses  of  ionized  air,  cloudlike  in  form, 
which  may  hover  in  the  body  of  the  upper 
atmosphere,  springing  into  existence  under 
the  influence  of  sunlight  and  disappearing 
when  the  ionizing  influence  of  sunlight  is  re- 
moved. It  is  also  thought  that  there  may  be 
a  close  connection  between  the  degree  of  at- 
mospheric absorption  and  the  amount  of  mag- 
netic variation  of  the  compass  needle,  judg- 
ing from  a  comparison  of  records  for  the 
daily  variation  of  transatlantic  electromag- 


204  WIRELESS  TELEGRAPHY 

netic  wave  mean  absorption,  and  for  the  reg- 
ularly tabulated  daily  variation  of  the  mag- 
netic needle. 

The  subject  of  atmospheric  absorption  of 
passing  electromagnetic  waves  *  is  a  promis- 
ing field  for  future  investigation  bearing  upon 
meteorology  and  terrestrial  magnetics. 

Transoceanic  Wireless  Telegraphy 

Regular  transatlantic  wireless  telegraph 
transmission  has  been  introduced  between 
Clifden  on  the  coast  of  Galway,  Ireland,  and 
Glace  Bay  on  the  shore  of  Cape  Breton  Is- 
land, Nova  Scotia,  an  oversea  distance  of  1930 
nautical  miles  (3570  kilometers  or  2220  statute 
miles).  Messages  exchanged  between  these 
terminal  antennas  are  transmitted  by  ordinary 
wire  telegraphy  to  other  places  on  each  side 
of  the  Atlantic  ocean.  It  is  stated  that  about 
5000  words  a  day  are  regularly  transmitted 
across  the  ocean  in  this  way. 

Extent  of   Use   of   Wireless   Telegraphy   on 
Vessels 

A  large  number  of  naval  vessels  of  different 
governments  are  now  equipped  with  wireless 

*  See  an  important  paper  on  "  Wireless  Telephony " 
by  R.  A.  Fessenden,  Proceedings  of  the  American  In- 
stitute of  Electrical  Engineers,  July,  1908. 


INDUSTRIAL  WIRELESS  TELEGRAPHY     205 

telegraph  apparatus.  Wireless  telegraphy 
played  a  conspicuous  part  in  the  naval  maneu- 
vers of  the  Russo-Japanese  war.  By  its  means 
a  blockade  was  sustained  of  Port  Arthur  for 
many  months  by  a  Japanese  fleet  at  a  safe 
anchorage  a  considerable  distance  away. 

Wireless  telegraphy  equipment  has  been 
placed  on  board  steamers  of  the  following 
lines  crossing  the  Atlantic  ocean: 

The  Anchor  Line  Co. 

The  Cunard  Steamship  Co. 

The  Norddeutscher  Lloyd  Co. 

The  American  Line  Co. 

The  Allan  Line  Co. 

The  Atlantic  Transport  Co. 

The   Compagnie   Transatlantique. 

The  Red  Star  Line. 

The  White  Star  Line. 

The  Hamburg- American  Line. 

The  Belgium  S.  S.  Line. 

The  Scandinavian  American  Line. 

The  Navigazione  Generale  Italiana. 

The  Austro- American  Line. 

About  116  vessels  of  this  transatlantic  fleet 
are  so  equipped.  There  are  two  powerful 
wireless  transmitting  stations  on  the  shores 
of  the  North  Atlantic  Ocean,  one  at  Cape 


206  WIRELESS  TELEGRAPHY 

Cod,  Mass.,  and  the  other  at  Poldhu  in  Corn- 
wall. It  is  becoming  customary  for  a  vessel 
with  a  long-distance  equipment  to  maintain 
communication  across  the  Atlantic  with  one  of 
these  stations  until  she  establishes  communi- 
cation with  the  other;  so  that  at  no  time  is 
she  outside  the  range  of  communication. 

A  wirelessly  equipped  vessel  leaving  New 
York  is  in  communication  with  the  station  at 
Sea  Gate,  N.  Y.  This  is  carried  until  it  is 
exchanged  for  communication  with  Babylon, 
L.  I.  This  is  carried  until  Sagaponack,  L.  I. 
Next  Nantucket,  Mass.,  is  taken,  then  Sable 
Island,  and  finally  Cape  Race.  Communica- 
tion for  the  exchange  of  messages  is  thus 
maintained  for  about  seventy  hours  after  leav- 
ing port,  each  of  these  stations  being  in  per- 
manent wire  communication  with  the  rest  of 
the  world. 

It  is  stated  that  in  the  year  prior  to  Jan.  31, 
1906,  these  American  stations  sent  and  re- 
ceived, with  ships,  altogether  15,000  messages 
comprising  over  200,000  words. 

Number  of  Land  Wireless  Stations 

According  to  a  report  of  the  U.  S.  Navy 
Department  there  are  this  year  (1908)  about 
468  land  wirdess  stations  in  different  parts  of 


INDUSTRIAL  WIRELESS  TELEGRAPHY     207 

the     world,     either     erected     or     projected, 
namely : — 


Belgium  

2 

Argentina  .  .  . 

.  5 

Hong-Kong  .. 

I 

Denmark  ..  .  .  . 

n 

Brazil  

.11 

China   

o 

Germany   

.71 

Hawaii  

7 

France  

T8 

Chili   

.     7 

Japan   

Great  Britain.  . 

46 

Colombia  

T 

Dutch  E.  Ind.. 

5 

Holland  

6 

Costa  Rica.  .  . 

.    2 

Asiatic  Russia. 

i 

Spain    

TO 

Mexico  

6 

Egypt   . 

/i 

Portugal    

T 

Panama  

.    2 

r^-^ 
Morocco  

c 

Gibraltar  

I 

United  States. 

III 

Mozambique  .. 

2 

Italy   

22 

Trinidad   .  .  .  . 

.    I 

Tripoli  

T 

Malta  

I 

Porto  Rico.  .  . 

.    2 

Canaries  

I 

Montenegro    .  . 

I 

San  Domingo. 

.    2 

Ecuador  

2 

Norway  

q 

Tongking  .  .  .  . 

.    2 

Formosa  

i 

Austria-Hung  . 

7 

Uruguay 

.    2 

T 

Finland 

2 

Zanzibar 

2 

Korea 

Switzerland  ..  . 

I 

Australia  

.  5 

Nicaragua  

2 

Roumania 

6 

Cuba 

10 

Peru 

c; 

Russia   

15 

Tobago  

.    T 

Philippines   .  .  . 

4 

Sweden 

4" 

li    India        . 

.     3, 

Qi  M  -p-l 

^ 

Turkey  

5 

Burma  

.    T 

According  to  the  same  navy  department 
list,  there  are  at  this  date  some  340  mercantile 
vessels  equipped  for  wireless  telegraphy,  in- 
cluding the  Atlantic  liners  already  referred  to, 
and  carrying  flags  of  the  following  countries: 


Belgium   10 

Germany 38 

France    8 

Great  Britain 86 


Holland    10 

Italy 10 

United  States 141 

Canada  27 

Japan 10 


The  total  number  of  recorded  ship  and 
shore  stations  is  thus  about  800,  exclusive  of 
many  warships  of  various  nations, 


208  WIRELESS  TELEGRAPHY 

Each  station  possesses  a  definite  "  code- 
name  "  or  "  call-letter,"  which  is  usually  a 
group  of  two,  or  even  three,  letters,  such  as 
BA  (Babylon,  L.I).  Stations  are  called  by 
their  code  names  and  they  sign  their  messages 
with  them.  This  is  even  more  necessary  in 
wireless  than  in  wire  telegraphy;  because  the 
distant  station  may  be  far  beyond  visible  range, 
and  be  otherwise  unidentified. 

It  is  customary  for  a  steamer  to  pick  up 
communication  with  another  steamer  at  a  dis- 
tance of  say  150  kilometers  (79.8  nautical 
miles,  or  93  statute  miles)  and  to  carry  on 
communication  until  the  distance  between  the 
ships  is,  say,  250  kilometers  (133  nautical 
miles,  or  155  statute  miles).  Occasionally, 
however,  messages  are  exchanged  between  a 
ship  and  shore  at  much  greater  distances  than 
these. 


CHAPTER  XV 

CONSIDERATIONS  PRELIMINARY  TO  WIRELESS 
TELEPHONY 

WIRELESS  telephony  depends  upon  the  same 
principles  as  wireless  telegraphy;  but  differs 
therefrom  in  details  connected  with  the  nature 
and  requirements  of  the  electric  telephone 
transmitter  and  receiver.  It  is  necessary, 
therefore,  to  have  a  sufficiently  clear  under- 
standing of  the  nature  and  mode  of  operation 
of  electric  telephony  with  wires,  in  order  to 
follow  understandingly  the  modified  nature 
and  mode  of  operation  of  electric  telephony 
without  wires. 

The  word  telephony  is  derived  from  two 
Greek  words  signifying  the  far-off  transmis- 
sion of  sound. 

Nature  of  Sound 

All  material  substances  are  capable  of  being 
compressed  and  dilated;  i.e.,  of  being  altered 
in  density,  by  the  application  of  suitable  forces 
209 


210  WIRELESS  TELEGRAPHY 

to  them;  although  the  facility  for  being  thus 
compressed  and  dilated  varies  enormously  in 
different  kinds  of  matter.  For  instance,  gases, 
such  as  the  air,  readily  admit  of  being  com- 
pressed or  dilated,  as  in  the  manipulation  of 
a  concertina;  while,  on  the  other  hand,  many 
liquids,  such  as  water,  require  so  much  more 
force  to  compress  or  dilate  them  that,  until 
the  year  1762,  it  was  supposed  that  water 
was  incompressible. 

When  a  body  is  subjected  to  a  rapid  rhyth- 
mic variation  in  its  density,  it  is  said  to 
vibrate,  as  when  the  metal  in  a  bell  is  forced 
into  rapid  small  alternations  of  compression 
and  dilation  by  a  hammer  blow;  or  when  the 
framework  of  a  building  is  set  into  slight  vi- 
bration by  the  passing  by  of  a  rapidly  moving 
railway  train.  Such  rapid  vibrations,  commu- 
nicating themselves  to  the  ear,  usually  by 
way  of  the  air  and  the  external  ear  orifice, 
excite  in  our  consciousness  the  sensation  of 
sound.  From  this  standpoint,  sound  is  a  par- 
ticular mode  of  sensation  excited  by  vibratory 
disturbances  of  density  in  neighboring  ma- 
terial substances.  But  it  is  also  customary  to 
call  a  vibratory  disturbance  sound,  when  this 
disturbance  is  capable  of  exciting  the  sensation 
of  sound.  Consequently,  sound  may  mean 


ELEMENTS  OF  WIRELESS  TELEPHONY      211 

either  the  particular  mode  of  physiological 
sensation  received  through  the  ear,  or  the  phys- 
ical disturbances  in  a  medium,  say  air,  such 
as  might  give  rise  to  the  sensation. 

Difference  Between  Musical  Sound  and  Noise 

When  the  vibratory  disturbance,  or  sound, 
in  a  material  medium  is  non-rhythmic  and  ir- 
regular in  repetition,  the  sensation  produced  is 
called  non-musical  sound,  or  noise,  as  when 
coal  is  emptied  from  a  cart  into  a  cellar.  The 
impacts  of  the  many  falling  lumps  of  coal  with 
the  cellar  floor,  or  with  each  other,  set  the 
coal  and  the  surrounding  air  into  powerful  in- 
coordinate vibration  of  a  jangling  character. 
When,  however,  the  vibratory  disturbance  is 
rhythmically  repeated,  the  sound  sensation 
produced  is  more  or  less  musical.  If  a  gong 
is  struck,  its  metallic  mass  is  thrown  into 
rapid  vibrations  which  may  be  readily  felt  by 
the  hand,  and  which  are  rhythmic  in  charac- 
ter. These  rapid  and  rhythmic  vibrations  are 
communicated  to  the  air  surrounding  the 
gong,  and  produce  alternate  compressions  and 
dilatations  in  the  air.  Such  local  disturbances 
in  the  density  of  the  air  do  not  remain  fixed 
in  the  space  where  they  are  produced;  but 
move  off.  in  all  directions  at  a  definite  speed. 


212  WIRELESS  TELEGRAPHY 

When  these  moving  air  pulses  impinge  upon 
the  eardrum  of  a  listener,  they  cause  the 
eardrum  to  vibrate,  and  communicate  to  the 
listener  the  sensation  of  a  musical  sound. 

The  Nature  of  Plane  Waves  of  Sound  in  a 
Speaking  Tube 

A  few  examples  of  the  spherical  expansion 
of  sound-waves  have  been  considered  in 
Chapter  II.  We  may  here  examine  the  par- 
ticular case  of  sound  transmission  within  and 
along  a  straight  pipe,  such  as  a  speaking-tube. 
In  Fig.  67,  A  A',  B  B',  represents  a  short 
length  of  a  speaking-tube  containing  air, 
through  which  a  simple  single  musical  note 
is  being  transmitted  acoustically.  The  actual 
process  is  quite  invisible;  but  we  may  repre- 
sent the  density  and  pressure  of  the  air  in  dif- 
ferent parts  of  the  tube  by  the  relative  prox- 
imity of  transverse  lines.  Where  the  density 
and  pressure  are  above  normal,  as  at  C  and  C', 
the  lines  are  heavy  and  closely  crowded. 
Where,  on  the  contrary,  the  density  and  pres- 
sure of  the  air  are  below  the  normal,  the  lines 
are  dotted  and  are  also  separated  out.  At 
the  particular  instant  selected,  as  represented 
in  the  figure,  the  air  at  the  points  C  C'  is  in 
the  state  of  maximum  compression;  while  at 


ELEMENTS  OF  WIRELESS  TELEPHONY      213 

D  D'  the  air  is  in  the  state  of  maximum  dila- 
tation. Between  NV  and  N2  the  air  is  mo- 
mentarily compressed;  while  between  N2  and 
N3  it  is  dilated  or  rarified,  to  a  correspond- 
ing degree;  but  with  ordinary  intensities  of 
sound,  the  differences  of  pressure  between  the 
compressed  and  dilated  portions  of  the  air  are 
remarkably  small. 

It  is  difficult  to  present  clearly  to  the  eye 
the  variations  of  density  and  crowding  in  the 
air  by  means  of  the  crowding  together  of 
transverse  lines  as  in  the  upper  part  of  Fig. 
67.  A  much  more  convenient  diagram  for 
this  purpose  is  given  in  the  lower  part  of  the 
figure;  where  the  straight  line  OO'  repre- 
sents normal  or  undisturbed  air-density  and 
pressure ;  while  deviations  above  this  line  stand 
for  compressions,  and  deviations  below  the 
line  for  dilatations  or  rarefactions.  Thus, 
the  wavy  line  d  %  c  n2  d'  n3  c'  indicates  the 
condition  of  pressure  and  density  of  all  the 
air  in  the  length  of  pipe  A  B,  at  the  instant 
selected. 

The  sound  waves  Nj.  C  N2  D  N3  C  do  not 
stand  still  but  move  along  the  tube,  from  the 
speaker  to  the  listener,  at  a  steady  speed,  which 
in  free  air  is  about  335  meters  per  second 
(noo  feet  per  second);  but  which  in  a  nar- 


214 


WIRELESS  TELEGRAPHY 


row  tube  may  be  somewhat  less,  owing  to 
friction  with  the  walls,  say  315  meters  per 
second  (1032  feet  per  second).  The  air  in 
the  tube  does  not  move  bodily.  When  a 
speaker  blows  through  a  speaking  tube,  the 
air  moves  bodily  along  it,  and  may  actuate  a 
whistle  when  escaping  through  the  distant 
end.  But  when  he  speaks  into  the  tube,  the 


FIG.  67. —  Diagram  Indicating  the  Comparative^Densities 
of  the  Air  in  a  Speaking  Tube  at  a  Particular  In- 
stant When  Transmitting  a  Single  Pure  Musical 
Tone. 

sound  of  his  voice  may  be  carried  through  the 
air  without  any  bodily  motion  of  that  air. 
Each  air  particle  joins  in  the  vibratory  mo- 
tion, and  oscillates  slightly  to  and  fro  in  the 
direction  of  the  tube's  length,  about  its  mean 
position  of  equilibrium.  In  other  words, 
when  sound  is  transmitted,  it  is  the  dis- 
turbance in  density  which  moves  bodily  along 
in  waves  and  not  the  parts  of  the  substance  in 


ELEMENTS  OF  WIRELESS  TELEPHONY      215 

which  the  disturbance  exists.  The  individual 
parts  merely  vibrate,  being  alternately  closer 
.together,  and  further  apart,  than  in  the  quies- 
cent state. 

Intensity  or  Loudness  of  Musical  Tones 

Musical  tones,  when  steadily  maintained  one 
at  a  time,  differ  from  each  other  in  intensity, 
and  in  pitch.  The  intensity  or  loudness  of 
the  sound  sensation  produced  by  a  simple 
musical  tone  depends  upon  the  amplitude  of 
the  vibration  producing  that  sensation ;  that  is, 
upon  the  maximum  excursion  of  the  air  par- 
ticles in  their  vibration  from  their  mean  po- 
sition of  equilibrium.  In  Fig.  67  the  ampli- 
tude would  be  measured  by  the  distance  o  c. 
The  loudness  of  a  sound  sensation  increases 
with  the  amplitude  of  vibration;  although 
not  in  simple  proportion.  The  sound  of  a 
steamer's  whistle  is  often  piercingly  loud  to 
a  listener  standing  on  the  deck  immediately  in 
front  of  it;  but  becomes  fainter  with  distance. 
This  means  that  the  amplitude  of  vibration 
of  the  particles  of  air  is  relatively  great  near 
the  whistle;  but  becomes  smaller  as  the  dis- 
tance from  the  whistle  increases,  and  as  the 
area  of  the  sound-wave  surface  expands. 
The  human  ear  is  so  sensitive  to  some  sounds, 


216  WIRELESS  TELEGRAPHY 

that,  according  to  accepted  measurements,  the 
sound  of  a  whistle  has  been  detected  in  air 
when  the  amplitude  of  disturbance  at  the 
listener's  ear  can  only  have  been  about  i  mil- 
limicron (i  m/x  or  ^_  inch.) 

Pitch  of  Single  Musical  Tones 

The  pitch  of  a  single  musical  tone  depends 
only  on  the  number  of  complete  vibrations,  or 
cycles,  of  disturbance  per  second.  A  note  of 
low  pitch,  like  that  of  a  deep  bass  voice,  pos- 
sesses relatively  few  vibrations  per  second; 
while  a  note  of  a  high  pitch,  like  that  of  a 
soprano  voice  in  its  upper  register,  possesses 
relatively  many  vibrations  per  second. 

The  number  of  complete  to-and-fro  vibra- 
tions, or  cycles,  of  vibration  per  second,  exe- 
cuted in  a  single  musical  tone  is  called  its  fre- 
quency, as  in  the  case  of  electric  vibrations 
referred  to  on  page  63.  A  note  of  high  pitch 
is,  therefore,  a  high-frequency  note,  and  a  note 
of  low  pitch  a  low-frequency  note.  The  hu- 
man ear  is  able  to  hear  sounds  whose  frequen- 
cies lie  between  about  16  cycles  per  second  in 
the  bass  and  about  16,000  cycles  per  second 
in  the  high  treble;  or  over  a  range  of  some 
ten  octaves,  the  limits  of  pitch  audibility  vary- 
ing to  some  extent  with  different  individuals. 


ELEMENTS  OF  WIRELESS  TELEPHONY      217 

The  usual  pianoforte  keyboard  is  from  At 
of  27  cycles  per  second,  in  the  bass,  to  c5  of 
4100  cycles  per  second,  in  the  treble,  or  about 
7^  octaves.  The  ordinary  range  of  pitch  in 
the  singing  voice  is  somewhat  less  than  2  oc- 
taves, a  man's  baritone  compass  being  com- 
monly from  A  of  108  to  f  of  316  cycles  per 
second,  and  a  woman's  soprano  compass  from 
c'  of  256  to  a"  of  854  cycles  per  second.  The 
fundamental  tone  of  a  man's  speaking  voice 
is  usually  in  the  neighborhood  of  150  cycles  • 
per  second  with  a  wave  length  in  air  of  about 
2  meters  (6.56  ft.),  and  that  of  a  woman's 
voice  near  300  cycles  per  second  with  a  wave 
length  of  about  i  meter  (3.28  ft.). 

Purity  of  Musical  Tone 

Contrary  to  what  might  be  supposed  at 
first  thought,  a  pure  musical  tone  in  the 
sense  of  a  single  simple  musical  tone,  cannot 
be  produced  by  the  human  voice,  is  very  diffi- 
cult to  produce  artificially,  and  wrhen  produced, 
is  not  particularly  pleasing  to  the  ear.  A 
musical  tone  produced  by  a  trained  voice  is 
found  to  be  not  a  single  simple  musical  tone 
of  the  desired  pitch ;  but  a  harmonious  associa- 
tion of  feebler  higher  pitch  tones  with  the  tone 
of  desired  pitch.  The  wave  form  of  a  sim- 


218 


WIRELESS  TELEGRAPHY 


pie  musical  tone  is  indicated  in  the  line  d  c 
d'  c'  of  Fig.  67.  A  close  approximation  to 
such  a  tone  may  be  produced  by  mounting  a 
tuning  fork  on  a  suitably  shaped  hollow  cham- 
ber, or  resonator.  A  flute  may  also  be  made 
to  produce  a  fair  approximation  to  a  single 


FIG.  68. —  Composition  of  a  Simple  Musical  Tone  with 
an  Overtone  of  Eight  Times  Its  Frequency  and 
One-Fifth  of  Its  Amplitude  Into  a  Resulting  Com- 
posite Musical  Tone. 

musical  tone.  Ordinarily,  what  we  describe 
as  a  single  musical  tone,  is  the  association  of 
that  tone  with  a  number  of  fainter  tones  of 
higher  pitch;  so  that  the  wave  shape  is  ren- 
dered complex.  To  take  a  simple  example, 


ELEMENTS  OF  WIRELESS  TELEPHONY     219 

Fig.  68  shows  at  o  A  B  c  D  the  wave-form  of 
a  certain  pure  musical  note,  say  middle  c'  of 
the  piano,  with  an  amplitude  Q  B.  Above  this 
appears  the  wave-form  of  another  pure 
musical  note  a  b  c  d  of  eight  times  the  fre- 
quency, and  corresponding  therefore  to  the 
triple  octave,  or  c""  above  the  treble  clef, 
with  an  amplitude  q  b,  one-fifth  of  Q  B.  If 
both  these  pure  musical  notes  are  sounded  to- 
gether, the  resulting  wave-form  is  shown  at 
A'  B7  C'  D'.  A  trained  ear  listening  to  the 
composite  note  might  detect  both  the  funda- 
mental tone  of  O  A  B  C  D  and  the  fainter 
overtone  or  harmonic  o  a  b  c  d,  which  would 
blend  together  harmoniously.  Even  an  un- 
trained ear  might  detect  that  the  quality  of  the 
composite  tone  A'  B'  C'  D'  was  different  from 
that  of  the  simple  tone  A  B  C  D. 

The   wave   shape   of   a   composite   musical 
tone  may  be  altered  in  three  ways-: — 

(1)  By  changing  the  number  of  associated 
overtones. 

(2)  By  changing  the   relative   amplitudes 
of  associated  overtones. 

(3)  By  changing  the  relative  positions,  or 
"  phases,"  of  the  overtones. 

The  quality  of  the  tone  as  appreciated  by 
the  ear  will  be  affected  by  changes  (i)  and 


220  WIRELESS  TELEGRAPHY 

(2),  but  not  by  (3).  In  regard  to  change 
(3),  it  may  be  observed  that  in  Fig.  68,  the 
overtone  has  the  negative,  or  downward, 
amplitude  a  at  the  moment  when  the  funda- 
mental tone  has  the  positive,  or  upward,  am- 
plitude A;  so  that  the  composite  tone  wave  is 
diminished  in  amplitude  at  A'  and  C';  but 
increased  at  B'  and  D/  If,  however,  the 
ripple  train  abed  were  advanced  though 
half  its  wave-length,  or  changed  in  phase  by 
half  a  wave,  with  respect  to  the  fundamental 
wave  O  A  B  C  D,  the  composite  tone  would 
have  the  same  quality  to  the  ear,  but  its  wave 
form  would  have  increased  amplitude  at  A' 
and  C',  with  diminished  amplitude  at  B' 
and  D'. 

It  usually  happens  that  a  source  of  musical 
tones,  such  as  a  horn,  trumpet  or  harp,  pro- 
duces, along  with  each  fundamental  tone,  an 
association  of  a  number  of  fainter  overtones, 
whose  frequencies  are  usually  all  simple  mul- 
tiples of  the  fundamental  frequency.  The 
number  and  relative  prominence  of  these 
overtones  give  the  distinguishing  quality  of 
the  note  produced  by  each  instrument.  Thus, 
a  flute  sounding  middle  c',  produces  relatively 
few  and  feeble  overtones.  The  fundamental 
tone  is  heard  almost  pure.  On  the  other 


ELEMENTS  OF  WIRELESS  TELEPHONY      221 

hand,  the  same  note  sounded  on  a  violin 
would  be  accompanied  by  a  large  number  of 
overtones,  of  successive  frequencies  2,  3,  4, 
etc.,  times  that  of  the  fundamental  note.  As 
a  general  rule,  the  higher  the  frequency  of 
an  overtone,  the  smaller  its  amplitude;  so  that 
beyond  a  certain  frequency  the  overtones  tend 
to  become  inappreciable.  Occasionally,  how- 
ever, particular  overtones,  such  as  the  7th  — 
or  Qth-frequency  overtones,  may  be  more 
prominent  than  their  neighbors.  The  shape 
and  physical  conditions  of  the  violin  sound- 
ing-board tend  to  accentuate  some  overtones 
more  than  others.  The  reason,  therefore, 
that  the  note,  say  "  middle  c',"  sounds  quite 
differently  when  sung  by  a  voice,  piano,  or 
violin,  lies  mainly  in  the  differences  of  associa- 
tions of  overtones,  and  in  the  corresponding 
wave  shapes  of  the  composite  tones. 

Moreover,  the  same  sustained  musical  note 
sung  by  a  trained  singer,  and  by  an  untrained 
singer,  may  be  very  different,  in  spite  of  the 
fact  that  each  may  be  producing  essentially 
the  same  fundamental  tone.  In  the  untrained 
voice,  there  is  likely  to  be  a  wavering,  or  .un- 
steadiness, of  pitch,  or  of  amplitude,  or  of 
both,  due  to  imperfect  muscular  control. 
There  may  also  be  an  unmusical  roughness, 


222  WIRELESS  TELEGRAPHY 

or  noise,  included  in  the  tone,  owing  to  the 
imperfect  interaction  of  the  vocal  chords  in 
the  larynx.  There  is  also  likely  to  be  an  un- 
pleasing  association  of  overtones,  both  in  re- 
gard to  their  relative  amplitudes,  and  to  their 
number;  while  the  particular  association  of 
overtones  may  be  varying,  or  wavering,  from 
moment  to  moment  in  an  unpleasant  manner. 
In  the  note  of  the  trained  singer,  we  are  likely 
to  find,  on  the  contrary,  a  sustained  steadiness, 
either  in  uniformity,  or  in  graded  change,  of 
loudness,  an  absence  of  roughness,  or  extra- 
neous unmusical  sound  (noise),  and  also  a 
pleasing  association  of  evertones,  brought 
about  by  the  habitual  formation  of  the  vocal 
cavities  so  as  to  resonate  with,  and  reenforce, 
harmonious  components.  Similarly,  the  dif- 
ference in  the  quality  of  the  same  notes  pro- 
duced by  a  player  successively,  and  with  the 
same  skill,  on  different  violins  or  pianos,  de- 
pends mainly  upon  their  respective  sounding- 
boards,  and  the  resonating  influence  of  these 
on  the  overtones.  Some  particular  blendings 
of  overtones  in  regard  to  number,  or  relative 
amplitudes,  are  more  pleasing  to  the  ear  than 
others.  The  skill  of  the  instrument  maker  is 
shown  in  the  resonating  qualities  he  is  able 


ELEMENTS  OF  WIRELESS  TELEPHONY     223 

to  bestow  upon  the  instrument  when  it  leaves 
his  hands. 

Tones  in  the  Speaking  Voice 

We  have  already  seen  that  the  wave-forms 
of  the  sounds  in  the  singing  voice  are  com- 
plex in  character,  owing  to  the  large  number 
of  different  single  tones  that  are  ordinarily 
contained  therein;  but  the  sounds  of  speech 
are  still  more  complex.  In  the  sounds  of 
speech  we  find  vowel-sounds  and  consonant- 
sounds,  as  well  as  inflections  and  cadences  of 
tone.  The  vowel-sounds  are  of  a  quasi-musi- 
cal character,  and  the  musical  quality  of  a 
speaker's  voice  depends  in  large  measure  upon 
them.  The  inflections  and  cadences  of  speech 
are  mainly  variations  in  the  fundamental  tones 
of  the  vowel-sounds.  The  consonant-sounds 
are  of  different  kinds,  such  as  labial,  dental, 
guttural  sounds;  but  are  mainly  quick,  sudden 
and  explosive.  The  more  prolonged  vowel- 
sounds  connect  and  are  terminated  by  the 
more  sudden  consonant-sounds.  The  defi- 
niteness  and  intelligibility  of  speech  resides 
principally  in  the  consonant-sounds.  Speech, 
deprived  of  its  consonants,  becomes  a  mere 
droning,  or  caricature  of  song. 


224  WIRELESS  TELEGRAPHY 

Some  of  the  consonant-sounds  are  feebler, 
or  have  smaller  amplitude,  than  vowel-sounds. 
This  is  particularly  the  case  with  sibilants, 
such  as  s,  z,  sst  etc.  One  of  the  most  difficult 
words  for  a  phonograph,  gramophone,  or  tele- 
phone to  repeat  is  "specie" 

Owing  to  the  large  range  of  frequency  in 
consonant-sounds,  and  their  frequent  lack  of 
amplitude,  it  is  more  difficult  to  reproduce 
articulate  speech  than  vowel-sounds,  or  music. 
It  may  be  possible  for  a  phonograph,  or  tele- 
phone, to  reproduce  recognizable  musical 
tones,  when  the  reproduction  of  recognizable 
speech  is  impossible. 


CHAPTER    XVI 


THE   PRINCIPLES    OF   WIRE    TELEPHONY 

IN  the  ordinary  process  of  electric  teleph- 
ony by  means  of  wires,  the  speaker  talks 
in  front  of  a 
"  transmitter  " 
such  as  that 
shown  at  T  in 
Fig.  69.  The  es- 
sential elements 
forming  this 
transmitter  are 
indicated  in  Fig. 
70.  M  is  a  hard 
rubber  mouth- 
piece, usually 

a  FIG.  69. —  Ordinary    Desk    Set    of 
a  y 


Telephone  Receiver  and  Trans- 
perforated     grid      mitter  as  Used  in  Wire  Teleph- 

at    the    base,    to      ony* 

prevent  a  pencil,  knife  or  other  pointed  in- 
strument    from    being    pushed     in,     to     the 
detriment    of    the    delicately    adjusted    parts 
225 


226  WIRELESS  TELEGRAPHY 

beyond.  An  aluminum  circular  diaphram  D 
is  supported  around  its  edge,  and  held  in  a 
soft  rubber  groove  or  gasket.  At  the  center 
C,  the  diaphram  is  hollowed  out  to  form  a 
circular  chamber.  In  this  chamber  are  placed 
two  carbon  disks  F  and  R,  separated  by 
granules  of  hard  carbon.  The  front  disk  F 
is  carried  by  the  diaphram  D.  The  rear  disk 
R  is  fastened  rigidly  to  a  pin  at  the  center  of 
the  solid  metal  back  B.  A  thin  mica  disk  A 
is  clamped  between  the  diaphram  and  the  rear 
disk,  so  as  to  close  the  chamber  flexibly  and 
_  ^^  maintain  a  mois- 

1  iPi  ibtate*  ture-tight  seal.  The 
front  and  rear  disks 
are  connected  b  y 
wires  to  the  termi- 
nals  of  the  trans- 
mitter. 

When  the  speak- 
er's  voice  is  directed 
towards  the  trans- 

FIG.  70.-  Sectional  View  of  mitter>  the  sound 
Principal  Parts  of  a  Tele-  waves  in  the  air  en- 
phone  Transmitter. 

ter    the    funnel    or 

mouthpiece  M,  and  impinge  upon  the  aluminum 
diaphram  D,  which  is  set  into  vibration  corres- 
ponding to  the  vocal  vibration.  The  diaphram 


ELEMENTS  OF  WIRELESS  TELEPHONY     227 

flexes  and  buckles  to  and  fro  very  rapidly,  as 
indicated  diagrammatically  by  the  dotted  white 
lines  in  the  figure.  Since  the  solid  metal  back 
plate  B  is  practically  rigid,  the  rear  carbon  disk 
R  stands  fixed,  and  resists  the  vibratory  force. 
Consequently,  the  particles  of  hard  carbon  C 
lying  between  the  vibrating  front  carbon 
disk  F  and  the  stationary  rear  carbon  disk  R, 
are  subjected  to  alternating  compression  and 
relaxation  of  pressure.  These  vibratory 
changes  in  pressure  accompany  the  vibrations 
of  the  diaphram  D,  which  as  we  have  seen, 
follow  the  air  vibrations  of  the  waves  of  sound 
arriving  from  the  speaker's  lips.  The  pow- 
dered carbon  F  has  the  peculiar  and  valuable 
property  that,  when  lying  loose  and  uncom- 
pressed, it  offers  considerable  resistance  or  ob- 
struction to  the  passage  of  an  electric  current; 
whereas,  when  compressed  and  compacted, 
this  obstruction  is  in  considerable  part  re- 
moved. Consequently,  a  voltaic  battery,  con- 
nected to  the  transmitter,  is  able  to  send  more 
current  through  the  powdered  carbon  F  each 
time  that  the  diaphram  D  is  moved  inwards 
to  compress  the  carbon,  but  is  compelled  to 
send  less  current  each  time  that  the  diaphram 
D  is  moved  outwards  to  release  pressure  on 
the  carbon.  Each  vibratory  motion  of  the 


228  WIRELESS  TELEGRAPHY 

disk  thus  produces  a  corresponding  vibratory 
impulse  of  electric  current  in  the  wires 
carrying  the  current  from  the  battery  to  the 
transmitter.  It  is  as  though  the  vibrating 
disk  governed  a  little  throttle-valve,  by  which 
electricity  was  alternately  admitted  to  and  cut 
off  from  the  circuit  comprising  the  battery, 
the  wires,  the  transmitter,  and  any  other  in- 
struments included  therewith. 

It  is  easily  understood  that  the  successive 
vibratory  movements  of  the  diaphram  are  im- 
mediately followed  by  similar  successive  elec- 
tric current  impulses  along  the  wires  con- 
nected to  the  transmitter,  owing  to  the  cor- 
respondingly varying  electric  resistance  of 
the  carbon  particles  F.  The  electric  current 
impulses  move  along  the  conducting  wires  as 
invisible  electromagnetic  waves  at  very  great 
speed.  If  the  diaphram  D  behaved  perfectly, 
it  would  faithfully  repeat  in  its  vibratory 
movements  each  and  all  of  the  vibratory 
movements  of  the  impinging  air  particles.  In 
other  words,  a  perfectly  acting  diaphram  D 
would  follow  all  of  the  faintest  ripples  on  the 
back  of  the  most  complex  wave-forms  per- 
taining to  the  incident  vocal  sounds.  As- 
suming such  perfect  behavior  on  the  part  of 
the  diaphram,  the  wave-forms  of  the  vibra- 


PRINCIPLES  OF  WIRE  TELEPHONY      229 

tory  pressure  communicated  to  the  powdered 
carbon  c  would  be  the  exact  counterparts  of 
the  wave-forms  of  the  vocal  sounds  uttered 
by  the  speaker.  The  effect  of  the  corre- 
sponding changes  in  electric  resistance  in  the 
carbon  would  be  to  produce  electric  currents 
whose  wave-forms  would  all  correspond  with 
those  of  the  vocal  sound-waves.  As  a  mat- 
ter of  fact,  however,  the  diaphram  D  is 
never  perfect  in  its  behavior.  It  tends  to 
develop  favorite  vibrations  of  its  own,  con- 
sidered as  a  flat  bell  or  gong,  and  it  distorts 
more  or  less  in  its  actual  vibrations,  the  wave- 
forms of  the  air  vibrations  impinging  on  its 
surface.  Nevertheless,  the  vibrations  of  the 
diaphram  D  follow  those  of  the  incident  sound- 
waves sufficiently  nearly  for  practical  tele- 
phonic purposes,  and  the  electric  current 
waves,  which  closely  correspond  to  the  dia- 
phram's  vibrations,  represent  the  vocal  sound- 
waves fairly  well. 

By  means  of  suitable  delicate  electromag- 
netic mechanism,  the  electric  current  waves 
in  a  wire  telephone  circuit  can  be  made  to 
photograph  themselves,  if  care  is  taken  to 
make  them  relatively  powerful.  With  this 
object  in  view  the  circuit  must  be  compara- 
tively short :  that  is,  it  must  not  include  many 


230  WIRELESS  TELEGRAPHY 

miles  of  wire,  the  instruments  must  be  ad- 
justed as  delicately  as  possible,  and  the  speaker 
must  place  his  lips  close  to  the  mouthpiece 
of  the  transmitter  and  speak  in  a  full  clear 
tone.  Many  persons  fail  to  make  themselves 
clearly  heard  in  ordinary  wire  telephonic  con- 
versation, because  they  talk  into  the  circum- 
ambient air,  instead  of  talking  into  the  trans- 
mitter. A  low  tone  of  voice,  with  the  lips 
nearly  touching  and  fully  opposite  to  the 
transmitter  mouthpiece,  is  likely  to  be  more 
effective  in  making  the  distant  listener  under- 
stand, especially  in  long-distance  telephony, 
than  loud  shouting  with  the  face  directed 
away  from,  or  to  one  side  of,  the  transmitter. 
Fig.  71  presents  three  "  oscillograms "  or 
photographs  of  the  electric  current  wave- 
forms in  the  articulation  of  the  three  syllables 
cur,  pea,  and  tea*  Except  for  the  vibratory 
imperfections  of  the  transmitter  diaphram, 
above  referred  to,  these  electric  wave  pictures 
may  be  regarded  as  portraits  of  the  sound- 
waves in  the  voice  of  the  speaker  that  uttered 
those  syllables.  Beginning  at  A  on  the  top 
line,  the  interval  A  B  represents  a  small  frac- 

*  From  a  paper  on  "  Telephonic  Transmission 
Measurements"  by  B.  S.  Cohen  and  G.  M.  Shepherd, 
Proceedings  of  the  Institute  of  Electrical  Engineers, 
London,  May,  1907. 


PRINCIPLES  OF  WIRE  TELEPHONY      231 

tion  of  one  second  of  time,  during  which  the 
speaker  uttered  the  syllable  cur.  First  comes 
the  c  consonant  or  sound  of  k,  as  a  train  of 
about  twenty  small  high-frequency  waves  of 
very  complex  form.  Then  there  is  a  brief 
pause,  during  which  the  muscular  adjustments 
appear  to  be  made  for  the  following  vowel- 
sound  ur,  and  finally  we  have  about  eight 


p    as    in    pea 
P 


FIG.  71. — •  Photographs  of  Electric  Current  Waves  in  the 
Transmission  of  Three  Particular  Vowel-Sounds. 


complete  fundamental  waves  of  the  vowel- 
sound,  judging  by  the  recurring  sharp  peaks 
below  the  line,  with  numerous  associated 
overtones  that  distort  the  fundamental  wave 
almost  beyond  recognition.  We  can  imagine 
that  if  the  outline  A  B  were  accurately  cut 
into  the  surface  of  a  wax  phonograph  cylinder, 
the  passage  of  the  reproducing  stylus  over  the 


232  WIRELESS  TELEGRAPHY 

indented  surface  might  cause  the  instrument 
to  repeat  this  syllable  cur. 

Similarly  with  the  syllable  pea,  as  recorded 
along  the  line  c  D.  First  comes  the  con- 
sonant sound,  then  a  brief  pause,  and  then 
about  eight  waves  of  the  fundamental  vowel 
sound  with  a  clearly  visible  prominent  over- 
tone ripple  of  perhaps  four  times  the  funda- 
mental frequency.  Again,  at  E  F,  in  the  syl- 
lable tea,  there  is  first  the  brief  explosive  con- 
sonant, then  a  pause  containing  apparently  a 
feeble  high  tone,  or  a  group  of  high-frequency 
tones,  and  finally  the  vowel-sound  which 
somewhat  resembles  the  vowel-sound  at  C  D. 

Changes  of  Wave-form  in  Telephonic  Trans- 
missions Over  Long   Wires. 

When  electromagnetic  waves  are  delivered 
to  a  pair  of  conducting  wires,  in  ordinary  wire 
telephony,  by  the  action  of  the  speaker's  voice 
on  his  transmitter,  two  changes  occur  in  these 
waves  as  they  are  carried  over  the  wires  to  the 
listener  at  the  receiving  end:  namely  (i)  a 
diminution  in  the  amplitude,  or  strength,  of 
the  waves,  and  (2)  a  different  diminution  in 
waves  of  different  frequency.  The  first 
change  is  a  mere  weakening,  like  that  of 
sounds  heard  at  great  distances  in  air.  It  is 


PRINCIPLES  OF  WIRE  TELEPHONY      233 

called  attenuation.  The  second  change  means 
that  the  different  frequency  components  in 
composite  sounds  are  attenuated  differently,  so 
that  the  shape  of  the  current  waves  arriving  at 
the  receiving  end  of  the  line  is  different  from 
that  of  the  outgoing  waves  at  the  sending 
end.  In  general,  tones  suffer  more  attenua- 
tion the  higher  their  frequency.  That  is,  the 


a 

FIG.  72. —  Oscillograms  of  Singing  Voice  at  Sending 
and  Receiving  Ends  of  a  Moderately  Long  Tele- 
phone Line. 


4 

FIG.  73. —  Oscillograms  of  Singing  Voice  at  Sending 
and  Receiving  Ends  of  a  Considerable  Length  of 
Telephone  Line. 

fundamental  tones  are  not  weakened  so  much 
as  the  over-tones.  The  result  is  that  the  char- 
acter of  the  transmitted  sound  is  altered  dur- 
ing the  electric  part  of  the  transmission. 

The  relative  influences  of  attenuation  and 
distortion  in  wire  telephony  are  fairly  well 
presented  in  Figs.  72  and  73,  .which  are  taken 


234  WIRELESS  TELEGRAPHY 

from  the  same  paper  as  the  last  illustration. 
In  Fig.  72,  A  B  is  the  oscillogram  of  the  elec- 
tric current  waves  at  the  sending  end  of  a 
telephone  line,  produced  by  a  fairly  high  note 
sung  into  the  transmitter  by  a  girl's  voice. 
The  corresponding  oscillogram  a  b  beneath, 
shows  the  electric  current  waves  at  the  re- 
ceiving end  of  the  line.  The  line  was  not  of 
great  length  from  a  telephonic  standpoint. 
In  the  oscillogram  A  B,  there  are  19  funda- 
mental waves,  judging  by  the  lower  peaks. 
These  correspond  to  the  frequency  of  the  note 
sung.  There  is  also  a  prominent  ripple  of 
three  times  the  fundamental  frequency,  and 
there  are,  besides,  other  overtones  discernible 
of  yet  higher  frequency.  In  the  oscillogram 
a  b  from  the  receiving  end,  there  are  the  same 
number  of  waves,  but  the  amplitude  is  re- 
duced. That  is,  there  is  evidence  of  consider- 
able attenuation.  Moreover,  there  is  evidence 
of  some  distortion,  because  the  outlines  of  the 
received  waves  are  not  merely  smaller  than 
at  A  B,  but  they  are  also  smoother  and 
rounder,  indicating  that  the  ripples  have  been 
attenuated  more  than  the  fundamental.  This 
is  more  clearly  shown  in  Fig.  73,  where  C  D 
and  c  d  are  the  oscillograms  at  the  sending  and 
receiving  ends  of  a  fairly  long  telephone 


PRINCIPLES  OF  WIRE  TELEPHONY      235 

circuit  when  the  syllable  oo  was  sung  into  the 
transmitter.  Here  14  waves  of  fundamental 
frequency  may  be  detected.  The  waves  re- 
ceived at  c  d  are  not  merely  attenuated.  If 
only  attenuated,  they  would  retain  the  exact 
shape  of  the  waves  of  the  sending  end,  on  a 
smaller  scale  of  amplitude.  They  have  also 
been  distorted.  The  sharp  overtones  and 
peaks  in  c  D  are  absent  in  c  d.  The  received 
waves  have  more  of  the  fundamental  and  less 
of  the  overtones  in  their  composition.  They 
approach  more  nearly  to  the  type  of  simple 
fundamental  wave  appearing  in  Fig.  67.  To 
a  listener  on  such  a  telephone  circuit,  the 
voice  of  the  speaker  might  be  intelligible;  but 
would  probably  sound  quite  differently.  It 
would  be  altered  in  character  and  would  prob- 
ably sound  "  drummy."  This  is  a  well 
known  condition  pertaining  to  wire-telephony 
over  circuits  that  are  electrically  very  long 
and  distorting. 

On  arriving  at  the  receiving  end  of  the  tele- 
phone circuit,  the  invisible  waves  of  electric 
current  are  enabled  to  reproduce  corre- 
sponding sound-waves  by  passing  through  the 
coils  of  fine  insulated  copper  wire  in  a  tele- 
phone receiver.  One  form  of  receiver  suit- 
able for  wearing  on  the  head,  has  already  been 


236 


WIRELESS  TELEGRAPHY 


described  in  connection  with  Figs.  50  and  51. 
A  particular  form  of  hand  receiver  is  seen 
partly  disassembled  in  Fig.  74.  The  hard 
rubber  shell  SS'  receives  the  connecting  wires 
at  the  narrow  end,  and  clamps  the  thin  ferro- 
type disk  'or  diaphram  D  between  its  broad 
end  S',  and  the  hard  rubber  screw  cover  R. 
Inside  the  shell  h  held  the  magnetic  system, 


FIG.  74.— Internal    Parts    of    a    Tele- 
phone Receiver. 


consisting  of  a  pair  of  hard  steel  permanently 
magnetized  bars  a  b,  c  d  connected  at  a  c  by  an 
iron  yoke-piece  and  terminating  at  b  d  in  a 
circular  bridge-piece  g  g  of  non-magnetic 
metal.  On  the  poles  b  d  are  mounted  soft  iron 
strips  forming  the  cores  of  two  small  electro- 
magnet coils,  which  are  wound  with  many 
turns  of  fine  silk-covered  copper  wire.  When 


PRINCIPLES  OF  WIRE  TELEPHONY      237 

assembled,  the  diaphram  D  is  clamped  close  to, 
but  out  of  contact  with,  the  soft  iron  pole- 
pieces  of  the  electromagnets.  These  are 
kept  magnetized  under  the  influence  of  the 
permanent  bar  magnets  a  b,  c  d;  so  that  the 
diaphram  D  is  steadily  attracted  or  pulled 
magnetically  towards  the  soft  iron  poles  when 
no  current  passes  through  the  instrument.  If 
now  a  current  passes  through  the  electromag- 
net coils  in  one  direction,  the  magnetic  pull 
of  the  permanent  magnet  is  strengthened,  thus 
tending  to  bend  down  or  buckle  the  diaphram 
D,  near  its  center.  If,  however,  a  current 
passes  through  the  coils  in  the  opposite  direc- 
tion, the  magnetic  pull  of  the  permanent  mag- 
net is  weakened,  and  the  elasticity  of  the  dia- 
phram D  tends  to  flatten  the  diaphram  or 
diminish  its  bending  down  at  the  center. 
Each  wave,  or  superposed  ripple,  of  electric 
current  sets  up  a  corresponding  up  and  down 
movement  of  the  center  of  the  diaphram,  the 
edge  of  which  is  held  fixed  between  the  ring 
on  S'  and  an  opposing  ring  on  the  cover  R. 
Rapidly  succeeding  current  waves  thus  throw 
the  diaphram  into  vibrations,  which,  if  the 
system  were  perfect,  would  be  identical  with 
those  of  the  transmitter  diaphram  at  the 
sending  end  of  the  circuit,  The  electromag- 


238  WIRELESS  TELEGRAPHY 

netic  vibration  of  the  receiver  diaphram  D  sets 
in  vibration  the  air  over  the  diaphram,  and 
when  the  cover  R  is  pressed  against  the 
listener's  ear,  the  sound-waves  are  led  through 
the  air  directly  from  the  diaphram  D  to  the 
eardrum.  Unless  the  electric  current  waves 
are  much  stronger  than  are  ordinarily  em- 
ployed in  telephony,  the  amplitude  of  vibra- 
tion of  the  diaphram  D  is  so  small  as  to  be 
imperceptible  except  with  the  aid  of  very  deli- 
cate instruments.  It  is  generally  less  than  i 
micron  (  ^~  inch).  Nevertheless,  within 
this  small  range,  the  vibratory  movement  of 
the  receiver  diaphram  corresponds  to  that  of 
the  transmitter  diaphram  under  the  influence 
of  the  speaker's  voice,  after  allowance  has 
been  made  for  the  electrical  and  mechanical 
imperfections  of  the  system. 


CHAPTER   XVII 

PRINCIPLES  OF  WIRELESS  TELEPHONY 

IN  order  that  long-distance  wireless  teleph- 
ony may  be  possible,  by  means  of  electro- 
magnetic waves  conducted  over  the  earth's 
surface,  it  is  necessary  that  an  antenna  at 
the  sending  station  should  radiate  waves  that 
are  definitely  related  to  the  sound-waves 
emitted  from  the  speaker's  lips,  and  that  an 
antenna  at  the  receiving  station  should  pick 
up  these  waves  and  utilize  them  in  such  a 
manner  as  to  reproduce  these  sound-waves. 

An  ideally  simple  arrangement  would  be 
that  the  transmitter,  actuated  by  the  speaker's 
voice,  should  generate  alternating  electric  im- 
pulses supplied  directly  to  an  antenna,  that 
the  antenna  should  radiate  the  energy  of  these 
impulses  in  electromagnetic  waves,  the  wave- 
forms of  which  would  be  identical  with  those 
of  the  actuating  vocal  sound-waves,  and  that 
the  receiving  mast-wire  should  be  connected  to 
ground  through  a  receiving  telephone,  and 
operate  the  same  by  the  electric  current  im- 
239 


240  WIRELESS  TELEGRAPHY 

pulses  produced  by  the  passage  of  the  waves. 
Such  an  arrangement  is,  however,  impracti- 
cable, because  the  power  of  the  human  voice  is 
insufficient  to-  generate  electromagnetic  waves 
capable  of  producing  audible  sounds  at  any 
considerable  distance.  Moreover,  the  fre- 
quencies that  are  serviceable  in  transmitting 
speech  are  relatively  low,  not  necessarily  ex- 
ceeding 2000  cycles  per  second,  and  seldom 
exceeding  5000  cycles  per  second.  An  an- 
tenna does  not  radiate  electromagnetic  waves 
to  any  considerable  extent  until  the  frequency 
is  raised  to  at  least  tens  of  thousands  of  cycles 
per  second.  Such  frequencies  extend  beyond 
the  limits  of  audibility. 

The  general  plan  that  is  adopted  is  to  supply 
electric  power  to  the  sending  antenna  under 
such  conditions  as  will  permit  of  sustained 
radiation  of  electromagnetic  waves.  This 
power  supply  is  modified  in  some  manner  by 
a  transmitter,  under  the  action  of  the 
speaker's  voice.  The  electromagnetic  waves 
radiate  out,  carrying  with  them  the  vocally 
imposed  modifications,  and  the  distant  re- 
ceiving antenna,  in  the  path  of  these  waves, 
is  able  to  make  their  modifications  audible  as 
articulate  sounds  in  the  connected  receiving 
telephone. 


PRINCIPLES  OF  WIRELESS  TELEPHONY      241 

Methods  of  Maintaining  Continuous  Radiation 
It  has  already  been  pointed  out  in  Chapter 
IX  that  when  a  sending  antenna  is  supplied 
with  power  from  an  induction  coil,  operated 
through  a  vibrator,  the  radiation  of  electro- 
magnetic waves  from  the  antenna  is  likely  to 
be  markedly  discontinuous.  For  instance,  if 
the  vibrator  delivers  200  electric  impulses  per 
second  to  the  antenna,  the  latter  may  radiate 
a  brief  train  of  waves  at  each  2OOth  part  of  a 
second,  with  relatively  long  intervening  gaps 
of  quiescence.  It  is  manifest  that  such  a  type 
of  radiation  is  ill  adapted  for  wireless  te- 
lephony, because  during  the  utterance  of  any 
one  syllable  at  the  transmitter,  there  will  be 
only  a  few  groups  of  waves,  emitted  from 
the  antenna  in  jets,  with  relatively  long  inter- 
vening pauses.  In  order  to  transmit  articu- 
late speech,  the  radiation  from  the  antenna 
must  be  continuously  sustained;  or,  if  discon- 
tinuous, the  discontinuities  must  be  relatively 
brief. 

Two  methods  have  recently  been  developed 
for  continuously  sustaining  the  radiation  from 
a  sending  antenna.  The  first  method  employs 
the  electric  arc.  The  second  method  employs 
a  high-frequency,  alternating-current,  dynamo 
machine. 


242 


WIRELESS  TELEGRAPHY 


The  Singing-Arc  Method  of  Setting  Up 
Sustained  Oscillation. 

One  variety  of  the  arc-lamp  method  is  rep- 
resented in  its  simplest  elements  by  Fig.  75. 
A  dynamo  D  supplies  direct  current  to  an  arc 
lamp  A,  through  suitably  adjusted 
electric  resistances  R  R',  and 
"  choking  coils  "  C  C'.  The  main 
function  of  the  resistances  is  to 
steady  and  control  the  current 
supplied  to  the  lamp;  while  the 
choking  coils  prevent  rapid  cur- 
rent oscillations  from  traversing 
the  branch  C  R  D  R'  C'  of  the 
system.  Connected  in  parallel 
with  the  arc  lamp  is  a  branch  cir- 


R 


D 


A  --; 


R'      C1 


FIG.  75  —  Arrangement  for  Maintaining  Continuous  Os- 
cillation of  an  Antenna  With  the  Aid  of  a  Voltaic 
Arc. 


PRINCIPLES  OF  WIRELESS  TELEPHONY      243 

cuit  c  P  c',  containing  condensers  c  c'  and  a  coil 
P,  which  also  forms  the  primary  winding  of  an 
induction  coil,  having  its  secondary  S  in  the 
sending  antenna.  The  condenser  capacity 
and  self-induction  of  the  branch  c  P  c'  are 
such  as  to  favor  the  production  of  suitable 
high-frequency  oscillations.  The  mast  wire  M 
is  also  tuned  to  the  same  frequency,  with  the 
aid  of  the  adjustable  coil  L,  as  described  on 
page  1 20,  in  connection  with  Fig.  37.  The  arc 
lamp  A  serves  to  excite  the  oscillating-current 
branch  c  P  c'  into  sustained  oscillatory  action, 
in  a  manner  about  to  be  described,  and  these 
oscillations  are  imparted  to  the  synchronously 
tuned  mast  wire  M,  through  the  induction 
coil;  so  that  the  mast  wire  is  kept  in  con- 
tinuous electric  oscillation,  and  therefore,  in 
steadily  sustained  radiation  of  electromagnetic 
waves.  The  energy  carried  off  by  these 
waves  is  supplied  by  the  dynamo  D  and  by  its 
prime  mover,  say  a  gas-engine  or  steam- 
engine. 

The  action  of  the  arc  lamp  by  which  it  ex- 
cites oscillatory  currents  in  the  electrically 
tuned  branches  c  PC'-  M  L  S  G,  is  somewhat 
complex  in  detail;  but,  in  outline,  is  simple 
enough.  The  solid  cylinders,  which  support 
the  arc  between  their  tips,  offer  comparatively 


244  WIRELESS  TELEGRAPHY 

little  resistance  to  the  passage  of  electric  cur- 
rent; but  the  vividly  incandescent  column  of 
metallic  vapor,  constituting  the  arc,  offers  a 
considerable  resistance,  which  depends  in  mag- 
nitude upon  the  strength  of  the  current  in  the 
arc.  If  the  current  is  feeble,  the  arc  is  a 
thin  band  of  incandescent  vapor,  and  offers 
a  relatively  high  resistance.  If  the  current 
through  the  arc  is  increased,  the  arc  itself 
swells  and  broadens,  while,  at  the  same  time, 
its  resistance  is  lowered.  In  other  words,  a 
stout  arc,  carrying  a  strong  current,  conducts 
electrically  better  than  a  thin  arc,  carrying  a 
weak  current. 

If  the  arc  lamp  A  is  started  with  the  oscilla- 
tory current  branch  c  P  of  removed  or  inter- 
rupted, a  steady  current  will  flow  from  the 
dynamo  D  through  the  arc  lamp  and  the  coils 
R  C,  R'  C'.  The  arc  will  burn  with  a  fairly 
steady  flame,  as  in  an  ordinary  street  arc 
lamp.  There  will  be  no  tendency  to  set  up 
high-frequency  alternating  currents  in  the  sys- 
tem. When,  however,  the  oscillatory  branch 
c  P  c'  is  applied  to  the  arc,  the  condensers  in 
this  branch  take  a  sudden  charge,  or  brief  cur- 
rent impulse,  which  is  deflected  from  the  arc, 
because  the  choking  coils  C  C'  do  not  permit 
a  sudden  change  of  current  to  occur  in  the 


PRINCIPLES  OF  WIRELESS  TELEPHONY      245 

supply  circuit.  The  sudden  diminution  of 
current  in  the  arc  instantly  causes  the  arc  to 
shrink  and  rise  in  resistance;  thus  tending  to 
throttle  the  current  in  the  supply  circuit 
D  R  C  A  C'  R'.  But  the  choking  coils  resist 
this  sudden  throttling  of  the  current,  and,  -in 
their  endeavor  to  keep  the  current  steady,  they 
force  electricity  into  the  condensers,  where  it 
can  go  for  the  moment,  after  the  conductive 
path  through  the  arc  is  obstructed.  The  con- 
densers thus  become  overcharged.  Their 
electric  elasticity  speedily  arrests  the  action, 
and  forces  electricity  back  through  the  arc, 
since  the  choking  coils  C  C'  resist  all  sudden 
changes.  The  current  now  builds  up  in  the 
arc,  and,  as  it  does  so,  the  arc  column  swells 
and  conducts  better.  The  arc  resistance  being 
thus  reduced,  the  condensers  over-discharge, 
being  aided  in  this  by  the  electromagnetic 
inertia  of  the  induction  coil  primary  winding 
P ;  while,  at  the  same  time,  a  strong  oscillatory 
impulse  is  delivered  from  ground  G  to  the 
mast  wire  M.  The  brief  over-discharge  of 
the  condensers  soon  terminates,  the  current 
in  the  arc  falls  to  the  normal  steady  value,  its 
resistance  rises,  and  current  is  thereby  de- 
flected again  into  the  condensers  so  as  to 
charge  them,  at  the  same  time  inducing  a 


246  WIRELESS  TELEGRAPHY 

reverse  impulse  in  the  mast  wire  in  synchro- 
nism with  its  natural  period  of  swing.  In  this 
manner  a  substantially  steady  flow  of  current 
is  delivered  by  the  dynamo  D,  but  alternately 
in  throbs  or  impulses  to  the  arc  A  and  to  the 
oscillatory  branch  c  P  c',  the  frequency  of  these 
impulses  being  determined  by  the  natural 
period  of  the  latter  branch. 

The  amplitude  of  the  current  oscillations 
that  can  be  imparted  in  this  way  to  the  oscil- 
latory current  branch,  and  to  the  mast  wire, 
depends,  other  things  being  equal,  upon  the 
change  in  resistance  of  the  arc  A  with  change 
of  current.  If  the  arc  changes  greatly  in 
resistance  for  a  given  change  in  current,  the 
action  above  described  will  be  powerful ;  while, 
if  the  arc  changes  but  little  in  resistance,  the 
action  will  manifestly  be  but  weak.  What  is 
needed,  therefore,  is  an  arc  that  is  very  sensi- 
tive in  its  resistance  to  changes  of  current. 
This  sensitiveness  is  found  to  depend  partly 
on  the  condition  of  the  solid  cylinders  sup- 
porting the  arc  between  their  tips,  and  partly 
on  the  condition  of  the  gas  in  which  the  arc 
is  formed. 


PRINCIPLES  OF  WIRELESS  TELEPHONY     247 

Means  Resorted  to  for  Increasing  the  Sensi- 
tiveness of  the  Singing  Arc 

It  has  been  found  that  the  sensitiveness  of 
the  arc  can  be  increased  by  substituting  for 
the  upper  carbon  rod  a  water-cooled  metallic 
cylinder,  and  also  by  substituting  for  atmos- 
pheric air  some  other  gaseous  medium  in 
which  the  arc  is  allowed  to  burn.  Both 
hydrogen  and  illuminating  gas  have  been  em- 
ployed. Moreover,  it  has  been  found  advan- 
tageous to  employ  a  plurality  of  sensitive  arc 
lamps  instead  of  a  single  arc  lamp,  in  order 
to  augment  the  action.  In  some  cases,  these 
arcs  are  all  connected  in  series,  while  in  others, 
they  have  been  all  connected  in  parallel. 

The  Singing  Arc 

The  sensitive  type  of  voltaic  arc  flame 
above  described  is  called  the  "  singing  arc," 
by  reason  of  a  curious  and  interesting  inverse 
property  which  it  possesses.  We  have  seen 
that,  when  properly  adjusted,  the  arc  auto- 
matically charges  and  discharges  the  associ- 
ated oscillatory  branch  c  P  c',  by  altering  its 
volume  and  conducting  power,  in  accordance 
with  rapid  variations  of  current  strength. 
Such  variations  of  breadth  and  volume  in  the 


248  WIRELESS  TELEGRAPHY 

arc  flame  set  up  corresponding  vibrations  in 
the  surrounding  air;  so  that  the  arc  is  able  to 
emit  sounds.  In  the  case  of  high-frequency 
alternations,  suitable  for  keeping  an  antenna 
in  electric  oscillation,  the  sound  would  prob- 
ably be  inaudible,  being  above  the  limits  of 
audibility;  but  if  the  frequency  is  sufficiently 
low,  the  arc  can  be  made  to  give  a  fairly  loud 
tone.  In  fact,  if  a  properly  adjusted  arc  lamp 
is  supplied  with  a  direct  current,  which  has 
passed  through  a  suitably  designed  carbon 
telephone  transmitter,  and  musical  sounds  im- 
pinge upon  the  transmitter  diaphram,  the  arc 
will  be  able  to  reproduce  them,  even  though 
the  arc  may  be  at  a  great  distance  from  the 
transmitter.  In  such  a  case,  the  transmitter 
produces  rapid  variations  in  the  current  sup- 
plying the  arc,  in  conformity  with  the  incident 
sound-waves.  These  current  variations  pro- 
duce corresponding  fluctuations  in  the  volume 
of  incandescent  vapor  in  the  arc,  which  there- 
by exerts,  in  its  turn,  corresponding  fluctua- 
tions in  the  pressure  upon  the  surrounding  air, 
and  so  produces  sounds  in  the  same.  An  arc 
lamp  can  thus  be  made  to  sing  and  reproduce 
music.  An  arc  so  adjusted  is  called  a  sing- 
ing arc.  It  is  even  possible  to  recognize  vo 
cal  sounds  reproduced  by  the  arc,  but  the  ar- 


PRINCIPLES  OF  WIRELESS  TELEPHONY     249 

ticulation  is  seldom  clean  In  the  case  -of 
adjusting  an  arc  to  reproduce  sounds,  the 
property  of  resistance  variation  in  the  arc 
vapor  accompanying  the  sound  is  not  brought 
into  service;  whereas  in  the  application  of  the 
singing  arc  to  exciting  an  antenna  into  oscil- 
lation, this  property  is  of  the  first  importance. 

Exciting  Sustained  Oscillation  in  an  Antenna 
by  Means  of  a  High-frequency  Alternator 

The  second  method,  mentioned  above,  for 
continuously  sustaining  the  radiation  from  a 
sending  antenna,  employs  a  specially  con- 
structed high-frequency  alternating-current 
dynamo,  or  alternator.  The  alternators  which 
are  used  in  America,  for  electric  lighting  and 
power  transmission  ordinarily  generate  either 
60  cycles  per  second,  or  25  cycles  per  second, 
the  former  frequency  being  suitable  for  street 
arc-lighting,  and  the  latter  for  power  trans- 
mission. The  natural  frequency  of  an  un- 
loaded antenna,  50  meters  (164  feet)  in 
height,  is  in  the  neighborhood  of  1,500,000 
cycles  per  second ;  so  that  this  would  be  the 
proper  frequency  that  an  alternator  should 
generate  in  order  to  be  in  simple  synchronism, 
or  in  resonance,  with  the  antenna.  By  load- 
ing the  antenna,  however,  as  described  in 


250  WIRELESS  TELEGRAPHY 

Chapter  IX,  it  is  possible  to  reduce  the  natural 
frequency  of  the  antenna,  or  the  frequency  of 
its  free  oscillation  and  radiation.  By  this 
means,  it  has  recently  been  found  possible  to 
bring  the  natural  frequency  down  to  a  limit 
which  specially  constructed  high-frequency 
alternators  can  attain. 


FIG.  76. —  High-Frequency  Turbo-Alternator. 

Fig.  76  illustrates  a  high-frequency  turbo- 
alternator  set.  On  the  right  hand  side  at  T 
is  a  de  Laval  steam  turbine  which,  with  the 
aid  of  step-up  gear,  drives  the  shaft  of  the 
alternator  A  at  a  speed  of  about  16,000  revo- 
lutions per  minute.  The  revolving  element, 
or  rotor,  of  the  alternator,  comprises  a  pair  of 


PRINCIPLES  OF  WIRELESS  TELEPHONY      251 

steel  disks,  in  the  periphery  of  which  300 
radial  groves  are  cut.  One  of  these  disks  is 
shown  in  Fig.  77.  Between  the  grooves  of 
two  such  revolving  disks  is  mounted  a  thin 
stationary  armature  frame  with  600  radial 
slots,  and  a  coil  in  each  pair  of  adjacent  slots, 
the  coils  being  then 
connected  in  series. 
Each  revolution  of  the 
disks,  with  their  300 
polar  teeth  or  projec- 
tions, produces  300 
cycles  of  alternating 
electromotive  force  in 

the  armature;  so  that  Fl£  77-—  One  of !  the  Two 

Revolving  Field   Poles  of 
at  the  Speed  of  l6,OOO       the    High-Frequency    Al- 

r.  p.  m.  there  will  be  ternaton 
generated  a  frequency  of  4,800,000  cycles 
per  minute  or  80,000  cycles  per  second.  If 
now  the  armature  is  connected  between  the 
sending  antenna  and  ground,  and  the  antenna 
is  tuned  to  this  frequency,  the  alternator 
will  be  producing  electric  impulses  in  step 
with  the  natural  electric  oscillations  of 
the  antenna,  and  the  system  will  be  brought 
into  full  swing.  The  power  developed  elec- 
trically at  the  alternator  terminals  will  be  all 
radiated  out  from  jthe  antenna  in  electromag- 


252  WIRELESS  TELEGRAPHY 

netic  waves,  after  deducting  the  heat  losses 
which  occur  by  the  up  and  down  movement  of 
the  electric  current  in  the  antenna.  It  has  been 
found  that  the  radiated  power  from  a  large 
and  powerful  sending  antenna,  when  excited 
to  resonance  in  the  above  manner,  represents 
a  load  on  the  high-frequency  alternator  such 
as  would  be  produced  by  a  simple  non-induc- 
tive resistance  of  8  or  10  ohms.  That  is,  the 
antenna,  when  in  full  oscillation,  behaves  as 
though  it  were  grounded  at  the  top  of  the 
mast  through  such  a  resistance. 

Receiving  Circuit  Connections 

When,  by  either  of  the  above  methods,  or 
by  some  other  arrangement,  a  sending  antenna 
is  excited  into  steady  radiative  action,  a  cor- 
responding steady  emission  of  electromagnetic 
waves  takes  place  at  this  antenna.  Any  re- 
ceiving mast  within  effective  range  will  then 
be  able  to  pick  up  a  steady  electric  disturb- 
ance in  the  antenna,  caused  by  the  continued 
passage  of  these  waves  as  they  run  by.  The 
disturbance  will  take  the  form  of  an  alternat- 
ing electromotive  force,  as  described  in  Chap- 
ter VIII,  and  the  frequency  of  the  alternation 
will  be  identical  with  that  of  the  sending  an- 
tenna. The  electrical  effect  of  this  alternat- 


PRINCIPLES  OF  WIRELESS  TELEPHONY      253 

ing  disturbance  will  be  a  maximum  when  the 
receiving  antenna  is  tuned  into  resonance  with 
that  frequency,  by  suitably  adjusting  its  load 
of  inductance,  or  capacity,  or  both.  It  may 
be  possible  to  use  any  type  of  sensitive  and 
rapidly  acting  wave  detector  in  the  receiving 
antenna  circuit  connected  with  a  receiving 
telephone.  An  electrolytic  receiver,  such  as 
that  described  in  connection  with  Fig.  46,  is 
found  to  answer  the  purpose  satisfactorily, 
and  the  receiving  connections  may  be  such  as 
are  indicated  in  Fig.  63.  Under  these  condi- 
tions, a  high-frequency  received  current  will 
pass  through  the  receiver  r,  and  a  current  of 
the  same  frequency  will  also  be  set  up  in  the 
coils  of  the  receiving  telephone  connected  to 
the  receiver.  The  frequency  of  even  a  heavily 
loaded  antenna  is,  howevef,  far  above  the 
highest  frequency  that  the  ear  can  detect;  so 
that  nothing  is  heard  in  the  receiving  tele- 
phone, although  a  very  considerable  high-fre- 
quency alternating  current  may  be  maintained 
flowing  up  and  down  the  receiving  antenna 
through  the  receiver  r.  In  order  to  break  this 
silence,  it  is  necessary  to  modify  the  oscilla- 
tions of  the  sending  antenna  in  accordance 
with  the  vocal  sound-waves  of  the  speaker, 
and  to  cause  these  modifications  in  the  emitted 


254 


WIRELESS  TELEGRAPHY 


waves  to  manifest  themselves  in  the  receiving 
telephone.  Fig.  78  indicates  diagrammati- 
cally  a  method  of  accomplishing  this  result. 
The  rapid  oscillations  O  A  B  C  D  E  F  repre- 
sent either  the  high-frequency  alternating  cur- 
rents supplied  steadily  to  the  sending  antenna 

A  B  °  D  E  F 


FIG.  78. —  Diagram  Illustrating  the  Production  of  Audi- 
ble-Sounds by  the  Modification  in  Amplitude  of 
Ultra-Audible  Frequency  Currents. 

when  no  telephonic  transmission  occurs,  or  the 
high-frequency  waves  which  are  steadily  being 
radiated,  under  that  condition,  from  the  send- 
ing antenna.  If  now  the  carbon  transmitter 
at  the  sending  station  can  be  made  to  alter 
the  amplitude  of  these  outgoing  waves, 
in  accordance  with  the  diagram  O'A'B'C'D'- 


PRINCIPLES  OF  WIRELESS  TELEPHONY      255 

E'F',  which  performs  periodic  variations  of 
thirty  times  lower  frequency,  then  the  tele- 
phone connected  to  the  receiving  antenna,  as 
indicated  in  Fig.  63,  may  be  regarded  as  giving 
vibrations  of  its  diaphram  corresponding  to 
the  variable  amplitude  high-frequency  waves 
A'B'C'D'E'F'.  This  high-frequency  may  be  be- 
yond the  limits  of  audibility ;  but  the  amplitude, 
wavering  at  the  lower  frequency,  may  pro- 
duce an  audible  effect  corresponding  to  the 
simple  musical  tone  wave  a  b  c  d  e  f. 

Conditions  Sufficient  for  the  Transmission  and 
Reproduction  of  Speech 

In  order,  therefore,  that  articulate  speech 
may  be  transmitted  from  the  sending  to  the 
receiving  antenna,  and  rendered  capable  of 
recognition  by  a  listener  at  the  latter,  it  is 
sufficient  that  an  alternating  current  of  ultra- 
audible  frequency  be  steadily  produced  in  the 
receiving  antenna,  and  its  apparatus,  when  the 
speaker  is  silent,  and  that  when  the  speaker 
talks  into  the  transmitter,  the  latter  shall  con- 
trol the  amplitude  of  the  high-frequency 
waves,  so  that  the  complex  wave-forms  of  the 
vocal  tones  may  be  developed  in  the  shapes  of 
the  waves  of  amplitude.  Under  these  condi- 
tions, if  the  degree  of  amplitude  affected  is 


256  WIRELESS  TELEGRAPHY 

sufficient,  and  the  distance  between  the  send- 
ing and  receiving  stations  is  not  too  great, 
the  receiving  telephone  may  reproduce  the 
vocal  tones  of  the  ^speaker  with  sufficient 
power  to  make  speech  recognizable.  In  the 
case  of  Fig.  78,  only  a  pure  musical  note 
could  be  expected  to  become  audible,  but  if 
the  transmitter  had  produced  any  association 
of  tones  within,  its  compass,  however  com- 
plex, the  same  association  might 
be  expected  to  be  bound  up  in 
the  rapidly  varying  amplitudes 
of  the  successive  outgoing  waves 
and  might  be  expected  to  be  re- 
produced by  the  receiving  tele- 
phone. 

One  method  of  controlling  the 
amplitude  of  the  high-frequency 
out-going  waves,  in  accordance 
with  lower  frequency  vocal 
sounds,  employs  a  carbon  trans- 
mitter in  the  main  sending  an- 
tenna path  as  indicated  in  Fig. 
FIG.  79.— Trans-  79-  In  this  case,  the  antenna 

A'""  iTn^a  M    havinS   been   adjusted    into 
Branch.  resonance    with    the    high-fre- 

quency alternator  A,  behaves  substantially  like 
a  non-inductive  resistance  to  ground,  that  is,  it 


M 


PRINCIPLES  OF  WIRELESS  TELEPHONY      257 

virtually  closes  the  circuit  of  the  alternator  upon 
the  radiation  resistance  of  the  antenna  plus  local 
connections.  The  carbon  transmitter  T,  in  the 
main  circuit,  alters  the  resistance  of  this  circuit 
in  conformity  with  the  vocal  sound-waves  im- 
pinging upon  its  diaphram.  The  amplitudes 
of  the  high-frequency  alternating  current,  and 
of  the  electromagnetic  waves  emitted  from  the 
antenna,  are  thus  caused  to  fol- 
low the  wave  forms  of  the 
speaker's  vocal  tones.  It  is  not, 
however,  necessary  that  the 
transmitter  should  be  inserted 
in  the  main  circuit.  The  trans- 
mitter may  be  placed  in  a  circuit 
which  is  inductively  connected 
with  the  main  circuit,  through 
the  medium  of  an  induction-coil 
such  as  is  shown  at  I  in  Fig.  80. 
A  condenser  has  also  been  used 
in  the  local  transmitter  circuit 
instead  of  the  voltaic  battery. 
In  such  cases,  the  variation  of  FIG.  80.— Trans- 
resistance  in  the  transmitter  cir-  mitter  in  In- 
.  .  .  ,.  ductively 

cuit   when  the  transmitter   dia-     Connected 

phram    is   disturbed   by   sound-     Circmt- 
waves  is  reflected  inductively  into  the  main  an- 
tenna path,  and  serves  to  control  the  ampli- 


258  WIRELESS  TELEGRAPHY 

tudes  of  the  emitted  radiant  waves.  Another 
plan  has  been  to  place  the  high-frequency  al- 
ternator in  the  primary  circuit  of  an  induction 
coil,  the  secondary  circuit  of  which  is  connected 
with  the  antenna  in  the  manner  indicated  in 
Fig.  36,  care  being  taken  to  bring  both  the  local 
oscillation  branch  A  c  B,  and  the  antenna,  into 
syntony  with  the  high-frequency  alternator, 
and  with  each  other.  This  causes  a  steady 
stream  of  radiated  waves  of  the  same  fre- 
quency to  be  thrown  out  from  the  sending 
mast  and  the  amplitudes  of  these  radiant 
waves  is  modified  in  conformity  with  the  vocal 
tones  of  the  speaker,  by  means  of  a  trans- 
mitter, connected  either  in  the  antenna  branch, 
or  in  some  branch  inductively  associated  with 
it. 

The  distance  to  which  wireless  telephony 
can  be  practically  carried  depends  upon  the 
amount  of  electric  power  that  can  be  con- 
trolled in  amplitude  of  current  by  the  trans- 
mitter at  the  sending  antenna  and  upon  the 
limiting  minimum  of  electric  power  which  can 
be  picked  up  from  the  radiated  waves  by  the 
distant  receiving  antenna  and  utilized  to  oper- 
ate the  listener's  telephone.  The  amount  of 
power  which  the  ordinary  carbon  transmitter 
controls  in  wire  telephony  may  be  less  than 


PRINCIPLES  OF  WIRELESS  TELEPHONY      259 

one  watt  (equivalent  to  the  power  expended  in 
lifting  about  J4  lb.  I  foot  high  per  second)  ; 
whereas,  in  wireless  telephony,  much  larger 
powers^must  be  developed  and  controlled  by 
the  transmitter,  on  account  of  the  scattering  of 
this  power  in  all  directions  from  the  sending 
antenna.  The  power  that  the  transmitter  is 
required  to  handle  in  long-distance  wireless 
telephony  may  be  hundreds  or  even  thou- 
sands of  watts.  The  ordinary  carbon  trans- 
mitter of  wire  telephony  could  not  carry 
such  an  amount  of  power  without  becoming 
overheated  and  deranged.  A  special  form  of 
carbon  transmitter  designed  to  carry  and  con- 
trol alternating  currents  up  to  15  amperes  is 
shown  in  Fig.  81.  The  mouthpiece  M  leads 
the  incident  sound-waves  to  a  metallic  dia- 
phram  which  carries  a  short  metallic  rod  fas- 
tened at  its  center.  The  rod  passes  through  a 
hole  in  a  metallic  terminal  plate  and  terminates 
in  a  platinum-indium  spade.  Vibrations  of 
the  diaphram  cause  the  spade  to  vibrate  in  a 
chamber  packed  with  carbon  particles,  and 
having  for  walls  two  metallic  terminal  plates, 
which  are  separated  by  the  white  insulating 
ring.  The  metallic  terminal  plates  are  con- 
nected to  the  terminals  T  T  and  the  spade  to 
the  terminal  t.  Water,  admitted  by  the  open- 


260 


WIRELESS  TELEGRAPHY 


ings    W    W,    is    circulated    through   the   two 
terminal  plates  to  keep  them  from  being  over- 


FIG.  81. —  A  Form  of  Water-Cooled  Carbon  Transmit- 
ter Employed  in  Wireless  Telephony. 

heated  by  the  powerful   alternating  currents 
passing  through  the  carbon. 

Freedom  of  Wireless  Telephony  from 
Distortion 

It  seems  evident,  both  from  the  principles 
and  the  practice  of  wireless  telephony,  that 
although  the  radiated  impulses  which  carry 
the  vocal  tones  are  subject  to  marked  atten- 


PRINCIPLES  OF  WIRELESS  TELEPHONY     261 

nation,  being  weakened,  not  only  by  the  absorp- 
tion of  the  waves  into  the  surface  of  the  im- 
perfectly conducting  land  and  sea,  but  also 
by  the  expansion  of  the  waves  into  ever-in- 
creasing areas;  yet  they  are  not  subject  to  the 
distortion  which  accompanies  their  transmis- 
sion in  wire  telephony,  particularly  when  the 
wires  are  placed  close  together  in  an  un- 
derground cable.  In  other  words,  the  sound- 
waves of  wireless  telephony  get  fainter  as  the 
range  of  transmission  is  increased  up  to  the 
limits  at  present  existing;  but  all  of  the  tones 
transmitted  become  fainter  in  the  same  pro- 
portion; so  that  there  is  no  indistinctness  pro- 
duced by  the  alteration  of  tone  quality. 

Range   of    Wireless   Telephone    Transmission 

The  greatest  distance  reported  at  present  for 
the  transmission  of  recognizable  wireless  te- 
lephony in  America  is  from  Brant  Rock,  Mass., 
to  Washington,  D.  C,  a  distance  of  657  kilo- 
meters (408  miles).  In  Europe,  the  greatest 
reported  distance  has  been  from  Monte  Mario 
at  Rome,  Italy,  and  a  vessel  off  the  coast  of 
Sicily  near  Trapani,  an  over-sea  distance  of 
over  500  kilometers  (300  miles).  Wireless 
telegraphy  is  still  young,  but  wireless  telephony 


262  WIRELESS  TELEGRAPHY 

is  younger  still;  so  that  the  limits  of  range 
to  which  the  human  voice  can  be  carried,  on 
electromagnetic  waves,  are  by  no  means  yet 
set.  It  would  seem  that  the  limits  lie  with 
the  amount  of  current  and  power  which  can 
be  handled  by  the  transmitter,  assuming  that 
no  further  improvements  are  made  in  direct- 
ing the  outgoing  beam  of  electromagnetic  ra- 
diation, in  the  antennas,  or  in  the  delicacy 
of  the  receiving  instruments.  In  one  sense, 
the  extension  of  the  present  range  from  a 
few  hundred  kilometers  to  the  antipodes,  or 
half  way  around  the  world  (20,000  kilo- 
meters, or  12,000  miles),  would  be  less  won- 
derful than  the  already  accomplished  feat  of 
reproducing  recognizable  speech  at  the  range 
now  attained;  because  the  extension  of  the 
range  of  speech  to  the  antipodes  is  a  matter 
of  degree;  whereas  the  achievement  of  wire- 
less telephony  to  a  range  of  even  100  kilo- 
meters (60  miles),  is  a  wonderful  acquisition 
in  kind. 

It  is  only  reasonable  to  expect,  however, 
that  the  range  of  possible  wireless  telephony 
will  be  less  than,  and  gradually  increase  to- 
wards, the  range  of  possible  wireless  telegra- 
phy, because,  in  wireless  telegraphy,  the  prob- 
lem of  communication  is  limited  to  producing 


PRINCIPLES  OF  WIRELESS  TELEPHONY     263 

any  recognizable  type  of  signal  that  can  be 
repeated  in  successive  periods  of  dots  and 
dashes,  whereas,  in  wireless  telephony,  the 
problem  of  communication  involves  the  more 
complex  condition  of  reproducing,  at  the  re- 
ceiving antenna,  waves  that  have  been  succes- 
sively modified  in  a  long  succession,  substan- 
tially in  accordance  with  the  sound-waves  of  a 
speaker's  voice,  either  at  the  sending  antenna, 
or  at  a  station  connected  electrically  with  the 
sending  antenna. 

Selectivity  of  Wireless  Telephony 

Just  as  it  is  possible  to  select  at  a  wireless 
receiving  telegraph  station  one  particular 
series  of  waves  emitted  from  a  particular  send- 
ing station,  to  the  exclusion  of  other  sending 
stations,  by  some  method  of  tuning;  so  it  is 
possible  to  select  at  a  wireless  receiving  tele- 
phone station,  one  particular  series  of  waves 
emitted  from  a  particular  sending  station,  to 
the  exclusion  of  other  sending  stations.  For 
instance,  if  A,  B,  C  and  D,  are  four  wireless" 
telephone  stations,  so  located  as-  all  to  lie 
within  each  other's  range  of  influence,  and  if 
A  desires  to  speak  with  B  exclusively ;  while 
C  desires  to  speak  with  D  exclusively,  it  will 
suffice,  for  A  and  B  to  communicate  in  the  fre- 


264  WIRELESS  TELEGRAPHY 

quency  of  say  80,000  cycles  per  second  (wave- 
length 3.75  kilometers),  and  for  C  and  D  to 
communicate  in  the  frequency  of  say  90,000 
cycles  per  second  (wave-length  3  1-3  kilo- 
meters). That  is,  not  only  the  generating 
source  (arc  lamps  or  alternator)  at  A  would 
be  tuned  to  80,000  ^;  but  the  sending  an- 
tenna system  of  A,  and  the  receiving  antenna 
system  of  B,  would  also  require  to  be  tuned 
to  this  frequency.  When  suitably  tuned  in 
this  manner,  waves  of  frequency  90,000  "\ 
would  fail  to  be  detected  by  B's  receiver,  and, 
therefore,  all  variations  in  the  amplitudes  of 
such  waves,  capable  of  reproducing  speech, 
would  be  cut  off  from  B's  telephone.  The 
telephone  at  B  would  only  hear  the  speaker  A, 
to  which  it  was  adjusted  in  syntony.  With 
sharply-tuned  antenna  systems,  it  would  be 
possible  for  a  number  of  such  telephonic  con- 
versations to  be  carried  on  selectively,  each 
employing  a  powerful  series  of  independently 
acting  electromagnetic  waves. 

Simultaneous  Speaking  and  Listening 

With  the  arrangements  above  described,  it 
would  be  necessary  to  employ  a  switch  to 
change  the  antenna  connections  of  a  wireless 
telephone  station  from  sending  to  receiving, 


PRINCIPLES  OF  WIRELESS  TELEPHONY      265 

i.e.,  from  speaking  to  listening,  alternately. 
With  ordinary  wire  telephony,  such  a  switch 
was  required  in  a  very  early  stage  of  the  art; 
but  no  switch  is  at  present  needed  for  this 
purpose,  the  transmitter  and  the  telephone  be- 


FlG.  82. —  Connections  for   Simultaneous   Speaking  and 
Listening.     Duplex  Telephony. 

ing  always  in  the  circuit  simultaneously  dur- 
ing conversation;  so  that  it  is  possible  both  to 
speak  and  to  listen  at  the  same  time.  An 
arrangement  of  connections  has  been  devised 
for  effecting  the  same  result  for  wireless  te- 
lephony, and  is  indicated  in  Fig.  82.  The  an- 
tenna A,  with  its  tuning  coil  1,  is  permanently 


266  WIRELESS  TELEGRAPHY 

connected  to  ground  G,  through  the  secondary 
windings  of  four  induction  coils,  i,  2,  3,  4  and 
an  artificial  antenna  L  c  R,  consisting  of  a  suit- 
ably adjusted  combination  of  inductance,  ca- 
pacity and  resistance.  The  high-frequency 
alternator  H  is  connected  to  the  transmitter 
T,  through  the  primary  windings  of  the  four 
induction  coils.  Under  these  conditions,  if  no 
sounds  are  delivered  to  the  transmitter,  a 
steady  high-frequency  alternating  current  is 
sent  through  the  four  induction  coils  to  both 
antennas.  The  real  and  artificial  mast  wires 
are  thus  both  thrown  into  full  electric  oscil- 
lation. If  the  artificial  antenna  is  properly 
adjusted  so  as  to  balance  the  real  antenna, 
the  four  induction  coils  will  mutually  neu- 
tralize each  other's  influences,  and  no  current 
will  flow  through  the  dotted  system  5,  c,  6,  c', 
which  connects  the  points  e,  f,  and  which  con- 
nects with  the  receiving  telephone  t.  Again, 
if  the  speaker  talks  into  the  transmitter  T, 
the  amplitudes  of  the  high-frequency  alter- 
nating currents  will  be  varied  in  accordance 
with  his  vocal  tones,  but  the  power  will  be 
equally  divided  between  the  real  antenna  A 
and  the  artificial  antenna  L  C  R.  The  power 
in  the  real  antenna  will  be  expended  in  radi- 
ated electromagnetic  waves,  after  deducting  in- 


PRINCIPLES  OF  WIRELESS  TELEPHONY     267 

cidental  losses  in  heating  the  mast  wires.  The 
power  in  the  artificial  antenna  will  be  ex- 
pended in  heating  the  resistance  R,  which  rep- 
resents a  radiation  resistance,  after  deducting 
incidental  losses  in  heating  L  and  C.  The 
real  antenna  may  attain  a  height  of  100  meters 
(328  feet),  or  more  and  may  cover  a  con- 
siderable area  of  ground  surface.  The  arti- 
ficial antenna  is  a  small  affair  that  may  be 
put  inside  a  cupboard  of  one  cubic  meter  space 
(26.4  cubic  feet). 

If  electromagnetic  waves  are  received  at  the 
real  antenna  A,  of  the  same  frequency  as  that 
to  which  it  is  tuned,  they  will  develop  an  al- 
ternating current  of  that  frequency  passing  to 
ground  from  the  antenna  through  the  points 
e  f,  and  the  dotted  receiving  system  between 
them.  Some  of  the  current  will  pass  also 
through  the  artificial  antenna  L  C  R;  but  will 
do  no  harm  except  in  weakening  the  effect  on 
the  receiver.  The  divided  primary  receiving 
system  50,  6c',  is  called  an  interference  pre- 
venter. All  of  these  four  elements  are  inde- 
pendently adjustable.  The  secondary  receiv- 
ing system  connects  the  two  secondary  coils 
through  the  liquid  barretter  or  wave  detector 
B,  which  is  also  connected  to  the  voltaic  bat- 
tery v  through  the  receiving  telephone  t.  By 


268  WIRELESS  TELEGRAPHY 

suitably  differentiating  the  two  oscillating- 
current  receiving  branches,  it  is  possible  to 
tune  the  secondary  system  to  respond  loudly 
to  the  selected  frequency,  and  to  the  practical 
exclusion  of  all  others.  The  result  is  that  the 
receiving  telephone  t  is  prevented  from  re- 
ceiving any  part  of  the  locally  generated  high- 
frequency  currents  passing  through  T,  owing 
to  the  differential  balance  between  the  four 
coils  1234  and  between  the  two  antennas 
A  and  L  C  R.  It  will  be  silent  to  those  cur- 
rents, whether  the  transmitter  T  is  spoken  to 
or  not;  but  the  receiving  telephone  t  is  able 
to  receive  the  incoming  electromagnetic  wave 
disturbances  reaching  the  real  antenna,  be- 
cause these  disturbances  are  not  destroyed  by 
the  differential  balance.  In  this  manner,  both 
speaking  and  listening  may  continue  simul- 
taneously, as  in  ordinary  wire  telephony,  al- 
though there  is  some  weakening  of  the  re- 
ceived currents,  and  also  half  the  power  avail- 
able for  sending  out  waves  is  absorbed  locally 
as  heat  in  the  artificial  antenna.  That  is, 
more  power  must  be  used  with  the  arrange- 
ment of  Fig.  82,  for  the  same  limiting  range 
of  recognizable  telephonic  communication,  than 
with  alternate  speaking  and  listening. 

The  system  of  connections  indicated  in  Fig. 


PRINCIPLES  OF  WIRELESS  TELEPHONY      269 

82  is  likewise  available  for  duplex  wireless 
telegraphy;  that  is,  for  the  simultaneous  send- 
ing and  receiving  of  messages  at  one  and  the 
same  antenna.  It  has  been  found  that  the  ar- 
tificial antenna,  once  adjusted  to  balance  the 
real  antenna,  requires  less  change  from. day 
to  day  than  does  the  "  artificial  line "  em- 
ployed in  duplexing  an  ordinary  wire  tele- 


FIG.  83. —  Adjustable    Condenser    and    Induction    Coil 
Forming  Elements  of  Interference  Preventer. 

graph  line.  This  is  .apparently  due  to  the  fact 
that  changes  of  weather  have  more  influence 
in  changing  the  electric  conditions  of  a  line 
hundreds  of  kilometers  in  length,  than  in 
changing  the  electric  conditions  of  an  an- 
tenna. 

An  adjustable  air-condenser  and  an  adjusta- 
ble induction-coil  for  use  in  an  interference 
preventer  is  shown  in  Fig.  83. 


270 


WIRELESS  TELEGRAPHY 


Relaying  Telephonic  Currents  to  and  From  an 
Antenna 

In  telephoning  wirelessly  from  a  ship  to  a 
ship,  or  between  a  wireless  shore  station  and 
a  ship,  the  persons  conversing  together  are 


FIG.  84. —  Automatic  Telephone  Relay. 

close  to  their  respective  antennas;  but  when 
one  of  the  persons  is  on  shore  at  some  place 
telephonically  connected  with,  but  remote 
from,  the  wireless  telephone  station  on  the  sea 
coast,  and  wishes  to  speak  to  a  person  on  a 
ship  within  range,  it  is  necessary  for  his  con- 


PRINCIPLES  OF  WIRELESS  TELEPHONY      271 

versation  either  to  be  repeated  by  the  operator 
at  the  coast  station  acting  as  intermediary; 
or  to  be  repeated  automatically  to  and  fro  by 
relays  at  the  coast  station.  Fig.  84  shows 
a  relay  designed  for  this  duty.  It  consists 
essentially  of  a  telephone  receiver  in  which  a 
little  vibratory  tongue  is  substituted  for  the 
usual  vibratory  diaphram.  The  tongue  dips 
into  a  trough  containing  carbon  particles  with 
water-cooled  walls,  arranged  substantially  as 
in  the  transmitter  of  Fig.  81.  The  apparatus 
is  in  fact  a  telephone  receiver  directly  operat- 
ing a  carbon  transmitter.  The  receiver  is  con- 
nected at  the  coast  station  to  the  incoming 
telephone  line  wire  circuit,  and  transmits  di- 
rectly to  the  antenna.  Another  relay  receives 
from  the  antenna,  and  transmits  back  to  the 
wire  telephone  circuit. 


INDEX  OF  SUBJECTS 


PAGE 

A 

Activity  of  antenna....   100 
Air,  Sound- Waves  in. .       5 
Alphabets,   Morse. ..."..  153 
Alternating-current    ac- 
companying    radiated 

wave    47 

Alternating-current   dy- 
namo   105 

Alternator,     High  -  fre- 
quency   249 

Ammeter    * 193 

Amplitude     of     audible 

sound    216 

Amplitude    of    receiver 

diaphragm  vibration..  238 
Amplitude     of     sound- 
waves       10 

Antenna  97 

Antenna,  Activity  of...  100 
Antenna,     Artificial     in 
duplex    telephony    or 

telegraphy   269 

Antenna,   Essential  ele- 
ments of 98 

Antenna,  loaded,  Inter- 
nal reflections  in 123 

Antenna,   Radiation  re- 
sistance of 252 

Antennas,  Cylindrical...  102 

Antennas,  Fan 103 

Antennas,  Harp 103 

Antennas,  Height  of...  158 
Antennas,  Insulation  of  157 
Antennas,  Inverted-cone  103 


PAGE 

Antennas,  Loaded 120 

Antennas,  Multiple-wire  101 
Antennas,  Single-wire..   101 

Arc,  The  singing 247 

Artificial  antenna  in  du- 
plex telephony  or  te- 
legraphy   269 

Atmospheric  absorption 
and  magnetic  varia- 
tion of  compass,  Pos- 
sible connection  be- 
tween   203 

Attenuation  of  tele- 
phonic currents 233 

Audible  sound,  Ampli- 
tude of  216 

Auditory  selection. .....   175 

Auxiliary  condenser. ...  115 

B 

Branch  circuit,  Oscilla- 
tory    242 


Capacity  of  condenser. .   116 
Circuit,    Local,    of    re- 
ceiver   127 

Circuit,  Portable. ... . . .  193 

Changes  of  wave  form 
in  telephonic  trans- 
missions over  long 

wires 232 

Classification  of  elec- 
tromagnetic waves , , ,  74 


274 


INDEX 


PAGE 

Coherers 125 

Coil,  Self-induction  of. .  122 
Comparison  of  receivers  145 

Compass  of  tones 220 

Condensation  of  fluxes 
toward      equatorial 

zone 61 

Condenser,  Auxiliary.. .  115 
Condenser,  Nature  of . .  116 
Conditions  sufficient  for 
the  wireless  transmis- 
sion and  reproduction 

of  speech , . . .  255 

Connections  for  duplex 

telephony  265 

Consonance,  Electric...   118 
Continuous       radiation, 
Methods  of  maintain- 
ing    241 

Counter  -  electromotive  - 
force  of  polarization.   139 

Current-detectors    124 

Cylindrical  antennas —  102 


D 


Decohering,  Mechanical  129 
Depolarizing     of     elec- 
trodes      138 

Detectors,  Current 124 

Detectors,  Electrolytic..   125 
Detectors,     Electromag- 
netic     125 

Detectors,  Thermal 125 

Detectors,  Voltage 124 

Deviation    of    waves 
from       hemispherical 

form 60 

Diameter    of    telephone 

relay    270 

Difference  between  mu- 
sical sound  and  noise.  211 
Diminution  of  intensity 
in  sound-waves 8 


PAGE 

Directing  wireless  tele- 
graph waves 186 

Discontinuity  of  coil- 
fed  antenna  oscilla- 
tions    113 

Disk  galvanometer 195 

Distortion  of  telephonic 
wave  currents  over 

wires  233 

Duplex  wireless  tele- 
graph, Connections 

for  265 

Dynamo,  Alternating- 
current  105 


Earth's  Surfaces,  Im- 
perfect conduction  of  48 

Eddy  currents  in  sur- 
face of  earth  or  sea. .  78 

Electric  and  magnetic 
fluxes,  Tensions  in. ..  26 

Electric  and  magnetic 
forces  in  moving 
waves 79 

Electric  consonance....  118 

Electric  conductorsj 
Resonance  in 90 

Electric  Field 25 

Electric  Flux  and  its 
properties 23 

Electric  Flux,  Energy 
of 24 

Electric  Flux,  move- 
ment over  conductors  29 

Electric  Flux,  Proper- 
ties of 23 

Electric  Flux,  Provi- 
sional hypothesis  con- 
cerning nature  of. ...  25 

Electric  oscillations, 
Skin  depth  of 105 

Electric  Resonance. ....    91 


INDEX 


275 


PAGE 

Electric  strength  of  in- 
sulators    117 

Electricity  and  magne- 
tism, Nature  of 16 

Electric  theory  of  mat- 
ter    68 

Electrodes   140 

Electrolyte,  Definition 
of  127 

Electrolytic  detectors. . .   125 

Electromagnetic  detec- 
tors    125 

Electromagnetics,  and 
optics  73 

Electromagnetic  waves 
and  polarized  light. ..  67 

Electromagnetic  waves 
Classification  of 74 

Electromagnetic  wave- 
detectors 124 

Electromagnetic  -  wave, 
Energy  of 33 

Electromagnetic  waves, 
Hemispherical 59 

Electromagnetic  waves, 
Measurement  of 191 

Electromagnetic  wave, 
Nature  of 31 

Electromagnetic  waves, 
Plane 75 

Electromagnetic  wave, 
Radiation  of 40 

Electromagnetic  waves, 
Speed  of 33 

Electromagnetic  waves, 
Spherical  64 

Electromagnetic  wave- 
trains  

Electromotive  force 85 

Energy  in  waves 4 

Energy  of  Electric  Flux    24 

Energy  of  Electromag- 
netic waves 33 

Energy  of  Magnetic 
Flux 21 


PAGE 

Energy  of  received  elec- 
tric oscillations 94 

Energy  of  Wind 14 

Equatorial  zone,  Con- 
densation of  fluxes 
toward 61 

Ether,  Assumption  of 
The 15 


Fan  antennas., 103 

Free  Ocean  Waves 3 

Freedom  of  wireless  te- 
lephony from  distor- 
tion    260 

Frequency,  Group 175 

Frequency,  Relations  to 
wave  length  and  peri- 
odic time 62 


Galvanometer,    H  i  g  h  - 

frequency 195 

Gashes  in  waves,  torn 
by  vertical  conduc- 
tors    85 

German  practice 185 

Group  frequency 175 

Guided  electromagnetic 
waves 35 

H 

Harmonics 219 

Harp  antennas 103 

Heights  of  antennas...   158 
Helix,  wave-measuring.   197 
Hemispherical     electro- 
magnetic waves 59 

High-frequency  alterna- 
tor    249 

High-frequency  galva- 
nometer   195 

Hot-wire  receivers 132 


INDEX 


PAGE 


Image,   Electric 38 

Impact  of  waves  against 
vertical  conductors ...  92 

Imperfect  Conduction  of 
earth  48 

Induction  coil  for  high 
voltage 107 

Industrial  wireless  te- 
legraphy    198 

Insulation  of  antennas.   157 

Insulators.  Electric 
strength  of  117 

Intensity  of  musical 
tones 215 

Intensity  in  sound- 
waves, Diminution  of  8 

Internal  reflections  in 
loaded  antenna 123 

Interference,  Preventer 
267,  269 

Inverted-cone  antennas.   103 

Ionizing  of  rarefied  air.   166 

K 
Krakatoa  explosion. ....     1 1 


Light,   Plane-polarized..     72 

Loaded  antennas 120 

Local  circuit  of  receiver  127 
Loudness      oJf     musical 
tones 215 

M 

Magnetic  and  electric 
fluxes,  Tensions  in.  . .  26 

Magnetic  and  electric 
forces  in  moving 
waves.., 79 


PAGE 

Magnetic  Flux  and  its 
Properties  18 

Magnetic  Flux  Created 
by  Moving  Electric 
Flux 30 

Magnetic  Flux,  Energy 
of  .. 21 

Magnetism  and  Elec- 
tricity, Nature  of 16 

Matter,  Electric  theory 
of  68 

Measurements  of  elec- 
tromagnetic waves...  191 

Mechanical  decohering.   129 

Methods' of  maintaining 
continuous  radiation.  241 

Micron,  a  unit  of 
length  68 

Microsecond,  Unit  of 
time 99 

Mnemonic  rules  for  di- 
rections of  fluxes  in 
waves 52 

Morse  alphabets 153 

Morse  inkwriter 146 

Multiple-wire  antennas.    101 

Multiple  wireless  teleg- 
raphy    183 

Musical  sound  and 
noise,  Difference  be- 
tween    211 

Musical  tones,  Intensity 

or  loudness  of 215 

Musical  tones,  Pitch  of  216 
Musical  tones,  Purity  of  217 

N 

Nature  of  sound 209 

Number   of   land   wire- 
less stations  in  1908. .  206 
Non-conductors,  Trans- 
parency of  , , . . , 79 


INDEX 


277 


PAGE 

o 

Ocean  steamers,  Wire- 
less telegraph  equip- 
ment on 205 

Ocean  Waves,  Free....     73 

Optics  and  electromag- 
netics    93 

Oscillating-current  gal- 
vanometer    196 

Oscillating  currents  in 
receiving  verticals 94 

Oscillator,  Simple  ver- 
tical    49 

Oscillatory  branch  cir- 
cuit   242 

Oscillograms  of  wire 
telephone  currents. . . .  230 

Overtones 219 

Overtones,  Phase  of....  219 

P 

Periodic  time,  relations 
to  wave-length  and 
frequency  . 62 

Phase  of  overtones 219 

Pitch  of  single  musical 
tones  216 

Plane  electromagnetic 
waves 75 

Plane-polarized  light...     72 

Plane  waves  of  sound 
in  a  speaking  tube...  212 

Polarized  light  and  elec- 
tromagnetic waves...  67 

Portable  circuit 193 

Possible  connection  be- 
tween atmospheric  ab- 
sorption and  magnetic 
variation  of  compass.  203 

Power  required  for 
wireless  telegraph 
sending 160 

Preventing  interference 
..................267,269 

Purity  of  musical  tones  217 


PAGE 


R 


Radiation  of  electro- 
magnetic waves 49 

Radiation  resistance  of 
antenna 252 

Range  of  recognisable 
wireless  telephonic 
transmission  ....  .258,  261 

Received  electric  oscil- 
lations, Energy  of....  94 

Receivers,  Comparison 
of  ...  145 

Receivers,  Wireless  tel- 
egraph    124 

Relations  between  wave- 
length, frequency,  and 
periodic  time 62 

Relay  for  use  in  te- 
lephony   270 

Relay,  Electromagnetic 
130,  170  171 

Resonance  in  electric 
conductors  . . . . 90 

Resonant  selection 176 


Selective    signalling 173 

Selectivity    of    wireless 

telephony   263 

Self-induction  of  coil.. .   122 
Sending  apparatus,  ele- 
ments of no 

Sending  keys 151 

Shadow  cast  by  electric 

conductors 80 

Short-range  apparatus..   168 
Signalling  range  on  sea 

and  land 86 

Simple   vertical   oscilla- 
tor      49 

Simultaneous       sending 

and  receiving 180 

Simultaneous     speaking 
and  listening..  , . . 264 


INDEX 


PAGE 

Singing  arc,  The 247 

Singing  arc,  Method  of 

sustaining  oscillation.  242 
Single-wire  antennas. ..  101 
Skin  depth  of  electric 

oscillations 105 

Sound,  Nature  of 209 

Sound-wave  Trains u 

Sound-waves,  A  m  p  1  i  - 

tude  of 10 

Sound-waves,     Diminu- 
tion of  intensity  in....       8 

Sound-waves  in  air 5 

Sound-waves,  Speed  of.       8 
Speaking  and  listening, 

Simultaneous    264 

Speaking    tubes,    Plane 

waves  of  sound  in.. . .  212 
Speaking  voice,  Tones 

in 223 

Speed    of    electro-mag- 
netic waves 33 

Speed  of  sound-waves. .       8 
Spherical       electromag- 
netic wave 64 

Spreading   of    wireless- 
telegraph  waves 162 

Sunlight,    Effect    on 
wireless  telegraphy  of  201 


Telephone  Receiver, 

Principles  of 236 

Telephone  receivers 147 

Telephone  transmitter, 

Principles  of 225 

Tensions  in  electric  and 

magnetic  fluxes 26 

Thermal  detectors 125 

Thermo-electric  force. .  195 
Thermo-galvanometer  .  194 
Tones  in  the  speaking 

voice  ....  4 223 


PAGE 

Total  number  of  re- 
corded ship  and  shore 
stations  207 

Transmitter  employed 
in  wireless  telephony.  260 

Trans-oceanic  wireless 
telegraphy  204 

Transparency  of  electric 
non-conductors 79 

Tuned  signalling 173 

U 

Unguided  electromag- 
netic waves 64 


Vertical  conductors,  Im- 
pact of  waves  against  92 

Vertical  oscillator,  Es- 
sential elements  of. ..  98 

Vertical  oscillator,  Sim- 
ple    49 

Voltage 108 

Voltage-detectors 124 

W 

Wave-detectors,  Elec- 
tromagnetic    124 

Wave  form  in  telephon- 
ic transmissions  over 
long  wires 232 

Wave-length,  Relations 
to  frequency  and  peri- 
odic time  62 

Wave-lengths,  Determi- 
nation of 191 

Wave-lengths,  in  ocean 
waves  4 

Wave-measuring  helix..   197 

Wave  trains,  Electro- 
magnetic    48 

Wind  energy  14 


INDEX 


279 


PAGE 

Wireless  stations  in 
1908,  Number  of  land  206 

Wireless-telegraph  in- 
terference    173 

Wireless-telegraph  re- 
ceivers    124 

Wireless-  telegraphy 
equipment  on  ocean 
steamers  205 

Wireless-telegraphy,  In- 
dustrial    198 

Wireless-telegraphy  on 
vessels 202 

Wireless-telegraph  sta- 
tions, Number  of 204 

Wireless-  telegraph 
waves,  Spreading  of. .   162 


PAGE 

Wireless  telephone  cir- 
cuit connections 256 

Wireless  telephonic 
range 258,  261 

Wireless  telephony, 
Freedom  from  distor- 
tion   260 

Wireless  telephony,  Se- 
lectivity of 263 

Wireless  telephony, 
transmitter  employed 
in 260 

Wireless  transmission 
and  reproduction  of 
speech,  Conditions 
sufficient  for 255 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 


Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 

ENGINEERING  LIBRARY 


APR 


1950 


D  21-100m-9,'48(B399sl6)476 


24190 


271151 


UNIVERSITY  OF  CALIFORNIA  lylBRARY 


